st: Rename styling test

Spaces in the name make it harder to run the test individually,
even with `meson-test` fixed to handle spaces.

Pick a name without spaces, and while at it, assign a suite.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3641>

src/st/meson.build

@@ -221,7 +221,8 @@ if get_option('tests') and have_x11_client
     link_with: libst
   )
 
-  test('CSS styling support', test_theme,
+  test('css-styling', test_theme,
+    suite: 'st',
     workdir: meson.current_source_dir(),
   )
 endif