tests: make the command tests work when python is not the one in the shebang

We test a couple ways of running the meson command, and double check
that the debug logs record the meson command used (including python).
But when running the meson command as an executable instead of as a
python script, we see whichever version `env` in the shebang sees.

Fix this by mocking the python command as well.

run_meson_command_tests.py

@@ -132,13 +132,17 @@ class CommandTests(unittest.TestCase):
         bindir = (self.tmpdir / 'bin')
         bindir.mkdir()
         (bindir / 'meson').symlink_to(self.src_root / 'meson.py')
+        (bindir / 'python3').symlink_to(python_command[0])
         os.environ['PATH'] = str(bindir) + os.pathsep + os.environ['PATH']
+        # use our overridden PATH-compatible python
+        path_resolved_meson_command = resolved_meson_command.copy()
+        path_resolved_meson_command[0] = str(bindir / 'python3')
         # See if it works!
         meson_py = 'meson'
         meson_setup = [meson_py, 'setup']
         meson_command = meson_setup + self.meson_args
         stdo = self._run(meson_command + [self.testdir, builddir])
-        self.assertMesonCommandIs(stdo.split('\n')[0], resolved_meson_command)
+        self.assertMesonCommandIs(stdo.split('\n')[0], path_resolved_meson_command)
 
     def test_meson_installed(self):
         # Install meson