Warnings always raised in WarningsRecorder

[hunse]

This ensures that if tests for warnings are run more than once,
the warning is still raised as expected.

[nicoddemus]

Thanks @hunse! 😄

FYI, I changed your test to use testdir in 2bbe709:

Because the test relies that two subsequent tests that use pytest.warns to capture the exact same warning, it is better to use testdir to ensure test execution occurs in the order we expect (which might be different with pytest-xdist or pytest-random for example).