pw_unit_test: Fix python test runners for Windows

Windows can get mad when trying to run a python script using subprocess.
This allows python test runner scripts to run on Windows.

Change-Id: Ib6fff68a3b45bb758e8eb36b7d501aa1f4ed592e
diff --git a/pw_unit_test/py/pw_unit_test/test_runner.py b/pw_unit_test/py/pw_unit_test/test_runner.py
index 7390692..0d395ae 100644
--- a/pw_unit_test/py/pw_unit_test/test_runner.py
+++ b/pw_unit_test/py/pw_unit_test/test_runner.py
@@ -137,6 +137,10 @@
 
             _LOG.info('%s: [ RUN] %s', test_counter, test.name)
             command = [self._executable, test.file_path, *self._args]
+
+            if self._executable.endswith('.py'):
+                command.insert(0, sys.executable)
+
             try:
                 process = await pw_cli.process.run_async(*command)
                 if process.returncode == 0: