Use threaded HTTP server for test harness. (#24748)

The test `test_offset_converter_pthread` hangs during the `fetch` call
to report results to the server. This seems to be caused by Chrome
opening an earlier connection to the server and leaving it open which in
turn causes pythons`SimpleHTTPRequestHandler` to stop responding to new
requests. Using the `ThreadingHTTPServer` allows python to keep
processing requests and fixes the hang.

More info on this issue can be found in:
https://bugs.python.org/issue31639 and the chrome issue
https://issues.chromium.org/issues/40978518.

Fixes #10232

Author: brendandahl

test/common.py

@@ -9,7 +9,7 @@
 from subprocess import PIPE, STDOUT
 from typing import Dict, Tuple
 from urllib.parse import unquote, unquote_plus, urlparse, parse_qs
-from http.server import HTTPServer, SimpleHTTPRequestHandler
+from http.server import ThreadingHTTPServer, SimpleHTTPRequestHandler
 import contextlib
 import difflib
 import hashlib
@@ -2290,7 +2290,7 @@ def log_request(code=0, size=0):
   # do not have the correct MIME type
   SimpleHTTPRequestHandler.extensions_map['.mjs'] = 'text/javascript'
 
-  httpd = HTTPServer(('localhost', port), TestServerHandler)
+  httpd = ThreadingHTTPServer(('localhost', port), TestServerHandler)
   httpd.serve_forever() # test runner will kill us