context: TestCauseRace failures

#!watchflakes
default <- pkg == "context" && test == "TestCauseRace"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestCauseRace
    x_test.go:866: Cause returned context canceled, want TestCauseRace
--- FAIL: TestCauseRace (0.00s)

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