glib/tests/fileutils: Make more use of g_assert_no_errno()
This commit is shared with !1724 (merged).
gio/tests/live-g-file: Use g_assert_no_errno()
gio/tests/appmonitor: Use g_assert_no_errno()
gio/tests/gsettings: Use g_assert_no_errno()
gio/tests/gsettings: Assert that g_chmod succeeds
gio/tests/gsettings: Assert that temporary directory ends up empty
If there are stray files left over, g_rmdir() will fail with ENOTEMPTY.
Spun out from !1724 (merged) because I realised there were more places where we could have used
g_assert_no_errno() (!1204 (merged)) than I had previously thought. The first commit is also in !1724 (merged).
The last two commits add new assertions. The others are just a better way to spell what we were already doing.