Previously, DeferredLinesWriter would collect MockMvc output from
every test that has executed. If a test eventually failed, the
output from every test up to including the one that had failed would
be output, rather than just the output for the test that has just
failed.

This commit clears the deferred lines after each test, thereby
ensuring that when a failure occurs only the lines from the failing
test are output.

Fixes gh-17551