flake: TestRealRunnerStdoutAndStderr* unit tests

[abayer]

I've seen a couple of these fail a few times recently on CI with this output:

=== RUN   TestRealRunnerStdoutAndStderrPaths
hello world
    runner_test.go:51: stderr: got: , wanted: hello world
--- FAIL: TestRealRunnerStdoutAndStderrPaths (0.02s)

...and...

=== RUN   TestRealRunnerStdoutAndStderrSamePath
    runner_test.go:78: got: 0, wanted: 24
--- FAIL: TestRealRunnerStdoutAndStderrSamePath (0.01s)

I have not been able to reproduce locally.

/kind flake

[lbernick]

Seeing this as well, I think it was added recently in #4882; @bradbeck would you mind taking a look at this? It might help to use a fake clock (example in taskrun reconciler tests) as opposed to a real one

[abayer]

From @dibyom on #5250

TestRealRunnerStdoutAndStderrPaths is testing out the expected stdout and stderr strings are written out to files. However, the writes happen asynchronously which means that sometimes the output that we get is empty leading to the test being flaky.

[dibyom]

One fix might be to have the test wait in a loop till the file has content before comparing (with a time limit of a few seconds say)

[wlynch]

Took a crack at this in #5261 since this blocked one of my other PRs 🙃 - would appreciate any feedback, in particular since this is going against what a comment in the original PR said not to do (but I think it's fine anyway?)