Fix flakey tests

[dscho]

We frequently get CI test failures in t5570.9 or t5570.10 on macOS, and in t5504.9 on Linux, and re-running the test "fixes" it.

The t5570 failures have been discussed on the list in this thread (and I feel a conclusion is close): https://public-inbox.org/git/20190208195459.GA32556@sigill.intra.peff.net/

I am not aware of any mentions of the t5504 on the Git mailing list, so that will be interesting.

[dscho]

See e.g. https://dev.azure.com/gitgitgadget/git/_build/results?buildId=2354&view=ms.vss-test-web.build-test-results-tab and https://dev.azure.com/gitgitgadget/git/_build/results?buildId=2353&view=ms.vss-test-web.build-test-results-tab

[dscho]

For lurkers: jk/no-sigpipe-during-network-transport is on its way to git.git's master, and we integrated it early via #2111.

[dscho]

I'll just close this for now, as the remaining flakey tests are not all that annoying (t5504 seems to fail on linux-clang every once in a while, but that's manageable).