meson: Run the test with Python 3

This is a patch from me and a patch from Mathieu Bridon squashed
together.

Signed-off-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Mathieu Bridon <bochecha@daitauha.fr>

src/compiler/glsl/glcpp/meson.build

@@ -64,8 +64,9 @@ if with_tests
   foreach m : modes
     test(
       'glcpp test (@0@)'.format(m),
-      find_program('tests/glcpp_test.py'),
+      prog_python,
       args : [
+        join_paths(meson.current_source_dir(), 'tests/glcpp_test.py'),
         glcpp, join_paths(meson.current_source_dir(), 'tests'),
         '--@0@'.format(m),
       ],