tests: Don't check for missing install_tag if GLib is a subproject

The test script checks the entire Meson build, not just GLib, so it will
fail if GLib is a fallback subproject within some larger project that
does not use `install_tag` as systematically as GLib does.

In particular, if the larger project has a very conservative minimum
Meson version (like for example dbus), it might not be possible to
add `install_tag` to it.

Resolves: https://gitlab.gnome.org/GNOME/glib/-/issues/3625
Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/meson.build

@@ -25,6 +25,7 @@ if have_bash
   endforeach
 endif
 
+if not meson.is_subproject()
   test(
     'check-missing-install-tag.py',
     python,
@@ -33,5 +34,6 @@ test(
     suite : ['lint', 'no-valgrind'],
     protocol : 'tap',
   )
+endif
 
 subdir('lib')