meson, tests: Use suites for test cases

They allows to filter tests better and so we can just launch tests with:
  meson test --suite [mutter/stacking|cogl|clutter] [single-test-name]

clutter/tests/conform/meson.build

@@ -78,7 +78,8 @@ foreach test : clutter_conform_tests
     install: false,
   )
 
-  test('clutter/conform/@0@'.format(test), test_executable,
+  test(test, test_executable,
+    suite: ['clutter', 'clutter/conform'],
     env: test_env
   )
 endforeach

cogl/tests/conform/meson.build

@@ -95,7 +95,8 @@ cogl_conform_unit_tests = custom_target('cogl-tests-conform-unit-tests',
   install: false,
 )
 
-test('cogl/conform', cogl_run_tests,
+test('conform', cogl_run_tests,
+  suite: ['cogl'],
   args: [
     cogl_config_env,
     libmutter_cogl_test_conformance,

cogl/tests/unit/meson.build

@@ -32,7 +32,8 @@ cogl_unit_unit_tests = custom_target('cogl-tests-unit-unit-tests',
   install: false,
 )
 
-test('cogl/unit', cogl_run_tests,
+test('unit', cogl_run_tests,
+  suite: ['cogl'],
   args: [
     cogl_config_env,
     libmutter_cogl_test_unit,

src/tests/meson.build

@@ -109,13 +109,15 @@ foreach stacking_test: stacking_tests
   )
 endforeach
 
-test('mutter/unit', unit_tests,
+test('normal', unit_tests,
+  suite: ['mutter/unit'],
   env: test_env,
   is_parallel: false,
   timeout: 60,
 )
 
-test('mutter/unit/headless-start', headless_start_test,
+test('headless-start', headless_start_test,
+  suite: ['mutter/unit'],
   env: test_env,
   is_parallel: false,
   timeout: 60,