Add unit tests for executable_file and mch_can_exe.

Caution: There is a pending test which should be implemented.

test/unit/os_unix.moon

@@ -1,26 +1,44 @@
-{:cimport, :eq, :ffi} = require 'test.unit.helpers'
+{:cimport, :eq, :ffi, :lib, :cstr} = require 'test.unit.helpers'
 
-cstr = ffi.typeof 'char[?]'
+-- os = cimport './src/os_unix.h'
-os = cimport './src/os_unix.h'
+os = lib
+ffi.cdef [[
+enum BOOLEAN {
+  TRUE = 1, FALSE = 0
+};
+int mch_isdir(char_u * name);
+int executable_file(char_u *name);
+int mch_can_exe(char_u *name);
+]]
+
+{:TRUE, :FALSE} = lib
 
 describe 'os_unix function', ->
+  setup ->
+    lfs.mkdir 'unit-test-directory'
+    lfs.touch 'unit-test-directory/test.file'
+
+    -- Since the tests are executed, they are called by an executable. We use
+    -- that to unit test several functions.
+    export absolute_executable = arg[0]
+
+    -- Split absolute_executable into a directory and the actual file name and
+    -- append the directory to $PATH.
+    export directory, executable = if (string.find absolute_executable, '/')
+      string.match(absolute_executable, '^(.*)/(.*)$')
+    else
+      string.match(absolute_executable, '^(.*)\\(.*)$')
+
+    package.path = package.path .. ';' .. directory
+
+  teardown ->
+    lfs.rmdir 'unit-test-directory'
+
   describe 'mch_isdir', ->
-    TRUE = 1
-    FALSE = 0
-
-    ffi.cdef('int mch_isdir(char * name);')
-
     mch_isdir = (name) ->
       name = cstr (string.len name), name
       os.mch_isdir(name)
 
-    setup ->
-      lfs.mkdir 'empty-test-directory'
-      lfs.touch 'empty-test-directory/test.file'
-
-    teardown ->
-      lfs.rmdir 'empty-test-directory'
-
     it 'returns false if an empty string is given', ->
       eq FALSE, (mch_isdir '')
 
@@ -36,5 +54,51 @@ describe 'os_unix function', ->
     it 'returns true if the parent directory is given', ->
       eq TRUE, (mch_isdir '..')
 
-    it 'returns true if an newly created directory is given', ->
+    it 'returns true if an arbitrary directory is given', ->
-      eq TRUE, (mch_isdir 'empty-test-directory')
+      eq TRUE, (mch_isdir 'unit-test-directory')
+
+  describe 'executable_file', ->
+    executable_file = (name) ->
+      name = cstr (string.len name), name
+      os.executable_file name
+
+    it 'returns false when given a directory', ->
+      eq FALSE, (executable_file 'unit-test-directory')
+
+    it 'returns false when the given file does not exists', ->
+      eq FALSE, (executable_file 'does-not-exist.file')
+
+    it 'returns true when given an executable regular file', ->
+      eq TRUE, (executable_file absolute_executable)
+
+    it 'returns false when given a regular file without executable bit set', ->
+      -- This is a critical test since Windows does not have any executable
+      -- bit. Thus executable_file returns TRUE on every regular file on
+      -- Windows. This test must probably differentiate between Windows and
+      -- other systems, which have an executable bit.
+      pending 'Test executable_file with a regular file which is not
+          executable. Need a solution which also works on Windows.'
+
+  describe 'mch_can_exe', ->
+    mch_can_exe = (name) ->
+      name = cstr (string.len name), name
+      os.mch_can_exe name
+
+    it 'returns false when given a directory', ->
+      eq FALSE, (mch_can_exe 'unit-test-directory')
+
+    it 'returns true when given an executable in the current directory', ->
+      old_dir = lfs.currentdir!
+      lfs.chdir directory
+      eq TRUE, (mch_can_exe executable)
+      lfs.chdir old_dir
+
+    it 'returns true when given an executable inside $PATH', ->
+      eq TRUE, (mch_can_exe executable)
+
+    it 'returns true when given an executable relative to the current dir', ->
+      old_dir = lfs.currentdir!
+      lfs.chdir directory
+      relative_executable = './' .. executable
+      eq TRUE, (mch_can_exe relative_executable)
+      lfs.chdir old_dir