meson: Add option flags to control test suites building

Now the `tests` meson option controls weather we should build all the test suites
while `core_tests` controls mutter tests.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/442

src/meson.build

@@ -875,6 +875,6 @@ pkg.generate(libmutter,
 
 subdir('compositor/plugins')
 
-if have_tests 
+if have_core_tests
   subdir('tests')
 endif