Fix race conditions in invoker_test - #6905

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I have a couple of questions about your changes, but they are mainly for my own learning. Lifting listener initialization to the test makes sense to me.

Note: the Projectsv2 flakey test is being fixed in #6909

I noticed something interesting while testing this branch in a codespace: port 16634 is always unavailable because the "real" gRPC server running on my codespace uses it. We should use a different port in a followup PR.

@cmbrose Thanks for this! Do you think this PR also addresses the intermittent failure that happened for the 2nd-to-last commit in this branch?

--- FAIL: TestRebuildContainerFailure (0.00s)
    invoker_test.go:256: error connecting to internal server: error connecting to internal server: failed to share remote port 16634: dial tcp 127.0.0.1:16634: connect: connection refused
FAIL
FAIL	github.com/cli/cli/v2/internal/codespaces/rpc	0.054s

@mislav it should - the very last commit is meant to fix it, although the commit message doesn't really explain that 😅 Definitely let me know if they start popping up again though!