[3.14] gh-146352: In Emscripten pyrepl test, pick port dynamically (GH-146375) (#146411)

Dynamically allocates the port for the pyrepl browser test, so that multiple
tests can run at the same time. Also allows the pyrepl test to honor the
CROSS_BUILD_DIR environment variable.
(cherry picked from commit 2be147e1e75022d66eecb80b46904ed61a7a574f)