gh-146352: In Emscripten pyrepl test, pick port dynamically

So we don't run into problems when we run multiple copies of the buildbot at
once.

Author: hoodmane

.gitattributes

@@ -112,3 +112,4 @@ Tools/peg_generator/pegen/grammar_parser.py         generated
 aclocal.m4                                          generated
 configure                                           generated
 *.min.js                                            generated
+Platforms/emscripten/browser_test/package-lock.json generated

Platforms/emscripten/__main__.py

@@ -820,6 +820,7 @@ def main():
     context = parser.parse_args()
     context.emsdk_cache = getattr(context, "emsdk_cache", None)
     context.cross_build_dir = getattr(context, "cross_build_dir", None)
+    context.check_up_to_date = getattr(context, "check_up_to_date", False)
 
     if context.emsdk_cache:
         context.emsdk_cache = Path(context.emsdk_cache).absolute()