Previously, the deferred line writing that is used, to print MockMvc
results to the console assumed that each DeferredLinesWriter would
only be used by a single thread at a time. This assumption does not
hold true when using JUnit 5's parallel test exection if the tests
running in parallel share an application context. This resulted in
a concurrent modification exception if one thread was adding lines
to the output while another was iterating over them.

This commit updates DeferredLinesWriter so that it uses thread local
storage for the deferred lines. This ensures that each List of lines
is only ever accessed by a single thread.

Closes gh-16179