test_scala_proto_server test is flaky on MacOS CI

[crt-31]

When CI was running for PR #1580, the MacOS CI job failed twice, then passed on the 3rd time. (no code changes between the runs).

The failed test is //test:Run__test_test_scala_proto_server. Not much info is given as to why it failed because I don't have access to the test logs.

Both failures were when the job ran on [bk-imacpro-4]. (The third time it passed when it was run on [bk-imacpro-19].)

Log capture is here: rules-scala-scala_build_4801_dot-slash-test-rules-scala-on-darwin-macos.log

Conjecture: Run__test_test_scala_proto_server is creating a little server and communicating on a hardcoded port... I wonder if that port might be already in use or something causing the test to fail.