Fix ExecTestCase to properly handle shell scripts

Use shell=True to let the shell parse and execute shebangs properly.
The previous approach of always using sys.executable broke shell script
tests since it tried to run .sh files with the Python interpreter.

Author: jelmer

python/subunit/__init__.py

@@ -907,12 +907,7 @@ def debug(self):
 
     def _run(self, result):
         protocol = TestProtocolServer(result)
-        # Explicitly invoke Python to run the script instead of relying on
-        # execute permissions, which may not be preserved during installation
-        import shlex
-
-        script_parts = shlex.split(self.script)
-        process = subprocess.Popen([sys.executable] + script_parts, stdout=subprocess.PIPE)
+        process = subprocess.Popen(self.script, shell=True, stdout=subprocess.PIPE)
         make_stream_binary(process.stdout)
         output = process.communicate()[0]
         protocol.readFrom(BytesIO(output))