On Windows data can be lost in buffers in case of abnormal program
termination, especially in process chains as seen due to flaky tests.
Therefore flushing all buffers manually should avoid this data loss.
In the curl tool we play the safe game by only flushing write buffers,
but in the testsuite where we manage all buffers, we flush everything.
This should drastically reduce Windows CI and testsuite flakiness.
Reviewed-by: Daniel Stenberg
Supersedes #7833 and #6064Closes#8516
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