Fix flaky "t0021.15 required process filter should filter data"

[dscho]

This test case seems to fail infrequently, most often in the linux-gcc job. See e.g. https://dev.azure.com/gitgitgadget/git/_build/results?buildId=9226&view=ms.vss-test-web.build-test-results-tab&runId=19716&resultId=100893&paneView=debug

[tgummerer]

https://public-inbox.org/git/20190820065625.128130-1-t.gummerer@gmail.com/

I think we're good to close this

[dscho]

Good call! It already made it to next: git@58166c2, so I expect it to make it to master relatively soon (which is when GitGitGadget usually closes PullRequests, so I kinda thought about the same for issues, although that's a more manual process).