[meson] Don't run subset tests in parallel

Hopefully resolves https://github.com/harfbuzz/harfbuzz/runs/695051808#step:6:700 for now

test/subset/meson.build

@@ -38,6 +38,7 @@ foreach t : tests
     # as the tests are ran concurrently let's raise acceptable time here
     # ideally better to break and let meson handles them in parallel
     timeout: 200,
+    is_parallel: false
     workdir: join_paths(meson.current_build_dir(), '..', '..'),
   )
 endforeach