testsuite: Set $PATH for introspection tests

This is necessary because Windows uses $PATH to search for DLLs and
meson isn't smart enough to figure out that this test needs access
to the DLL.

testsuite/introspection/meson.build

@@ -3,6 +3,10 @@ env.prepend('GI_TYPELIB_PATH',
   project_build_root / 'gtk',
 )
 
+if host_machine.system() == 'windows'
+  env.prepend('PATH', project_build_root / 'gtk')
+endif
+
 test('api',
   find_program('api.py', dirs: meson.current_source_dir()),
   suite: ['introspection'],