Flush OutputStream-based appenders in Log4J 2 Tests

CI has been failing intermittently with failures in
Log4J2LoggingSystemTests. A possible cause of the failures is that
log entries are being buffered in an output stream. This may cause
an expected log entry to be absent (the entry is buffered) or an
unexpected entry to be present (the previously buffered entry has
now been flushed).

This commit attempts to address the test failures by ensuring that
all OutputStream-based appenders are flushed at the end of every test.

Closes gh-1864