Generate a lua module to help pass build-related settings.

This allows us to avoid hard-coding paths and using environment
variables to communicate key information to unit tests, which fits
with the overall goal of making sure that folks driving CMake directly
can continue to do out-of-tree builds.

test/config/paths.lua.in

@@ -0,0 +1,11 @@
+local module = {}
+
+module.include_paths = {}
+for p in ("${TEST_INCLUDE_DIRS}" .. ";"):gmatch("[^;]+") do
+  table.insert(module.include_paths, p)
+end
+
+module.test_include_path = "${CMAKE_BINARY_DIR}/test/includes/post"
+module.test_libnvim_path = "${TEST_LIBNVIM_PATH}"
+
+return module