Add infrastructure for testing origin policy

This adds 99 new subdomains for testing origin policy on, without interfering with other tests. It also configures `/.well-known/origin-policy` to use a Python script handler, instead of a static file handler, to allow varying responses. Closes #20773.

RFC: web-platform-tests/rfcs#44

tools/serve/serve.py

@@ -366,6 +366,7 @@ def add_mount_point(self, url_base, path):
             ("GET", "*.any.serviceworker.html", ServiceWorkersHandler),
             ("GET", "*.any.worker.js", AnyWorkerHandler),
             ("GET", "*.asis", handlers.AsIsHandler),
+            ("GET", "/.well-known/origin-policy", handlers.PythonScriptHandler),
             ("*", "*.py", handlers.PythonScriptHandler),
             ("GET", "*", handlers.FileHandler)
         ]
@@ -742,6 +743,9 @@ def build_config(override_path=None, **kwargs):
 def _make_subdomains_product(s, depth=2):
     return {u".".join(x) for x in chain(*(product(s, repeat=i) for i in range(1, depth+1)))}
 
+def _make_origin_policy_subdomains(limit):
+    return {u"op%d" % x for x in range(1,limit)}
+
 
 _subdomains = {u"www",
                u"www1",
@@ -753,6 +757,12 @@ def _make_subdomains_product(s, depth=2):
 
 _subdomains = _make_subdomains_product(_subdomains)
 
+# Origin policy subdomains need to not be reused by any other tests, since origin policies have
+# origin-wide impacts like installing a CSP or Feature Policy that could interfere with features
+# under test.
+# See https://github.com/web-platform-tests/rfcs/pull/44.
+_subdomains |= _make_origin_policy_subdomains(99)
+
 _not_subdomains = _make_subdomains_product(_not_subdomains)