tests: Add description of test directory structure and test env vars

test/README.md

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+# Tests
+
+Tests are run by `/cmake/RunTests.cmake` file, using busted.
+
+## Directory structure
+
+Directories with tests: `/test/benchmark` for benchmarks, `/test/functional` for 
+functional tests, `/test/unit` for unit tests. `/test/config` contains `*.in` 
+files (currently a single one) which are transformed into `*.lua` files using 
+`configure_file` CMake command: this is for acessing CMake variables in lua 
+tests. `/test/includes` contains include files for use by luajit `ffi.cdef` 
+C definitions parser: normally used to make macros not accessible via this 
+mechanism accessible the other way.
+
+Files `/test/*/preload.lua` contain modules which will be preloaded by busted, 
+via `--helper` option. `/test/**/helpers.lua` contain various “library” 
+functions, (intended to be) used by a number of tests and not just a single one.
+
+`/test/*/**/*_spec.lua` are files containing actual tests. Files that do not end 
+with a `_spec.lua` are libraries like `/test/**/helpers.lua`, except that they 
+have some common topic.
+
+Tests inside `/test/unit` and `/test/functional` are normally divided into 
+groups by the semantic component they are testing.
+
+## Environment variables
+
+Test behaviour is affected by environment variables. Currently supported 
+(Functional, Unit, Benchmarks) (when Defined; when set to _1_; when defined, 
+treated as Integer; when defined, treated as String; !must be defined to 
+function properly):
+
+`GDB` (F) (D): makes nvim instances to be run under `gdbserver`. It will be 
+accessible on `localhost:7777`: use `gdb build/bin/nvim`, type `target remote 
+:7777` inside.
+
+`GDBSERVER_PORT` (F) (I): overrides port used for `GDB`.
+
+`VALGRIND` (F) (D): makes nvim instances to be run under `valgrind`. Log files 
+are named `valgrind-%p.log` in this case. Note that non-empty valgrind log may 
+fail tests. Valgrind arguments may be seen in `/test/functional/helpers.lua`. 
+May be used in conjunction with `GDB`.
+
+`VALGRIND_LOG` (F) (S): overrides valgrind log file name used for `VALGRIND`.
+
+`TEST_SKIP_FRAGILE` (F) (D): makes test suite skip some fragile tests.
+
+`NVIM_PROG`, `NVIM_PRG` (F) (S): override path to Neovim executable (default to 
+`build/bin/nvim`).
+
+`CC` (U) (S): specifies which C compiler to use to preprocess files. Currently 
+only compilers with gcc-compatible arguments are supported.
+
+`NVIM_TEST_MAIN_CDEFS` (U) (1): makes `ffi.cdef` run in main process. This 
+raises a possibility of bugs due to conflicts in header definitions, despite the 
+counters, but greatly speeds up unit tests by not requiring `ffi.cdef` to do 
+parsing of big strings with C definitions.
+
+`NVIM_TEST_PRINT_I` (U) (1): makes `cimport` print preprocessed, but not yet 
+filtered through `formatc` headers. Used to debug `formatc`. Printing is done 
+with the line numbers.
+
+`NVIM_TEST_PRINT_CDEF` (U) (1): makes `cimport` print final lines which will be 
+then passed to `ffi.cdef`. Used to debug errors `ffi.cdef` happens to throw 
+sometimes.
+
+`NVIM_TEST_PRINT_SYSCALLS` (U) (1): makes it print to stderr when syscall 
+wrappers are called and what they returned. Used to debug code which makes unit 
+tests be executed in separate processes.
+
+`NVIM_TEST_RUN_FAILING_TESTS` (U) (1): makes `itp` run tests which are known to 
+fail (marked by setting third argument to `true`).
+
+`LOG_DIR` (FU) (S!): specifies where to seek for valgrind and ASAN log files.
+
+`NVIM_TEST_CORE_*` (FU) (S): a set of environment variables which specify where 
+to search for core files. Are supposed to be defined all at once.
+
+`NVIM_TEST_CORE_GLOB_DIRECTORY` (FU) (S): directory where core files are 
+located. May be `.`. This directory is then recursively searched for core files. 
+Note: this variable must be defined for any of the following to have any effect.
+
+`NVIM_TEST_CORE_GLOB_RE` (FU) (S): regular expression which must be matched by 
+core files. E.g. `/core[^/]*$`. May be absent, in which case any file is 
+considered to be matched.
+
+`NVIM_TEST_CORE_EXC_RE` (FU) (S): regular expression which excludes certain 
+directories from searching for core files inside. E.g. use `^/%.deps$` to not 
+search inside `/.deps`. If absent, nothing is excluded.
+
+`NVIM_TEST_CORE_DB_CMD` (FU) (S): command to get backtrace out of the debugger. 
+E.g. `gdb -n -batch -ex "thread apply all bt full" "$_NVIM_TEST_APP" -c 
+"$_NVIM_TEST_CORE"`. Defaults to the example command. This debug command may use 
+environment variables `_NVIM_TEST_APP` (path to application which is being 
+debugged: normally either nvim or luajit) and `_NVIM_TEST_CORE` (core file to 
+get backtrace from).
+
+`NVIM_TEST_CORE_RANDOM_SKIP` (FU) (D): makes `check_cores` not check cores after 
+approximately 90% of the tests. Should be used when finding cores is too hard 
+for some reason. Normally (on OS X or when `NVIM_TEST_CORE_GLOB_DIRECTORY` is 
+defined and this variable is not) cores are checked for after each test.