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>

docs/reference/glib/glib-sections.txt

@@ -3459,6 +3459,7 @@ g_assert_true
 g_assert_false
 g_assert_null
 g_assert_nonnull
+g_assert_no_errno
 g_test_set_nonfatal_assertions
 
 GTestCase