API: Test: Setup basic test infrastructure

- Add a 'expect' utility script that can run simple API tests using clients
  developed for any platform.
- Extend travis build matrix to run API tests using the python client and
  valgrind.

This script can be used to write API tests without having to manage nvim's
lifetime:

- It starts a single nvim instance listening on a known socket
- Invokes the test runner, which should connect to NEOVIM_LISTEN_ADDRESS
- The nvim instance started by the script provides a `BeforeEachTest` function,
  which should be called before each test to reset nvim to a clean state.
- It takes care of shutting down nvim once the tests are finished.

As explained
[here](https://github.com/neovim/neovim/pull/737#issuecomment-43941520), it's
not possible to fully reset nvim to it's initial state, but the `BeforeEachTest`
function should be enough for most test cases. Tests requiring a fully clean
nvim instance should take care of starting/stopping nvim.

scripts/run-api-tests.exp

@@ -0,0 +1,86 @@
+#!/usr/bin/env expect
+
+if {$argc < 2} {
+  puts "Need commands for running the tests and for starting nvim"
+  exit 1
+}
+
+set timeout 10
+set run_tests [split [lindex $argv 0] " "]
+set run_nvim [split [lindex $argv 1] " "]
+
+# don't echo to stdout
+log_user 0
+# set NEOVIM_LISTEN_ADDRESS, so nvim will listen on a known socket
+set env(NEOVIM_LISTEN_ADDRESS) "/tmp/nvim-[exec date +%s%N].sock"
+# start nvim
+spawn {*}$run_nvim
+# save the job descriptor
+set nvim_id $spawn_id
+# Reset function that can be invoked by test runners to put nvim in a cleaner
+# state
+send {
+:function BeforeEachTest()
+  set all&
+  redir => groups
+  silent augroup
+  redir END
+  for group in split(groups)
+    exe 'augroup '.group
+    autocmd!
+    augroup NONE
+    exe 'augroup! '.group
+  endfor
+  tabnew
+  let curbufnum = eval(bufnr('%'))
+  redir => buflist
+  silent ls!
+  redir END
+  let bufnums = []
+  for buf in split(buflist, '\n')
+    let bufnum = eval(split(buf, '[ u]')[0])
+    if bufnum != curbufnum
+      call add(bufnums, bufnum)
+    endif
+  endfor
+  if len(bufnums) > 0
+    exe 'silent bwipeout! '.join(bufnums, ' ')
+  endif
+  silent tabonly
+  for k in keys(g:)
+    exe 'unlet g:'.k
+  endfor
+  filetype plugin indent off
+  mapclear
+  mapclear!
+  abclear
+  comclear
+endfunction
+:echo "read"."y"
+}
+# wait until nvim is ready
+expect "ready"
+# run tests
+spawn {*}$run_tests
+set tests_id $spawn_id
+set status 1
+# listen for test output in the background
+expect_background {
+  * {
+    # show test output to the user
+    send_user -- $expect_out(buffer)
+  }
+  eof {
+    # collect the exit status code
+    set spawn_id $tests_id
+    catch wait result
+    set status [lindex $result 3]
+    set spawn_id $nvim_id
+    # quit nvim
+    send ":qa!\r"
+  }
+}
+# switch back nvim and wait until it exits 
+set spawn_id $nvim_id
+expect eof
+exit $status