gtestutils: Add a new g_assert_no_errno() test macro
This is for use in testing POSIX-style functions like rmdir(), which
return an integer < 0 on failure, and return their error information in
errno.
The new macro prints errno and g_strerror (errno) on failure.
Includes a unit test.
Signed-off-by: Philip Withnall withnall@endlessm.com