tests: Update g_assert()s in fileutils test to be more descriptive

Use the new g_assert_{non,}null(), g_assert_cmpint(), g_assert_true(),
etc., to get more descriptive output when the tests fail.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=782237