testsuite: force plugin tests sequentially on Windows.
Summary: Package registration does not seem to be thread-safe on Windows. Placing the system under heavily load seems to trigger registration failures even though they are all different package-dbs. This makes the plugin tests a bit flaky. I think this is because on Windows we use pessimistic locks while on Linux we use atomic file replacement. On Windows ReplaceFile is atomic, just the metadata write may not be. Since the metadata is not of importance we should either switch over to ReplaceFile or fix the locking code to not error out but wait. For now however I have to force these 25 tests to run serially in order to guarantee their correctness. Test Plan: ./validate Reviewers: bgamari Subscribers: rwbarton, thomie, carter GHC Trac Issues: #15313, #13194 Differential Revision: https://phabricator.haskell.org/D4918
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