tests: Remove redundant old atomic test

The test in `glib/tests/atomic.c` does everything this test did, and
more.

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

Helps: #1434

tests/meson.build

@@ -19,7 +19,6 @@ tests = {
   'testglib' : {'tap' : true},
   'testgdate' : {},
   'datetime' : {},
-  'atomic-test' : {},
   'bit-test' : {},
   'child-test' : {},
   'completion-test' : {},