Add another unit test to mch_can_exe.
Move mch_can_exe, executable_file to os/fs.c.

* Rename executable_file to is_executable.
Declare is_executable as static and remove its unit tests.

Testing the public interface mch_can_exe should suffice. Every former
test of is_executable has a counterpart in the tests of mch_can_exe.
Thus we can keep private things private.
Simplify mch_can_exe extracting is_executable_in_path.