Add Log4j 2.x agent tests for the 'log4j2' domain

Add Log4j 2.x agent tests for the 'log4j2' domain

Add integration tests for the new Log4j 2.x agent in its native mode
using the new 'log4j2' domain, the new configure switch
'--enable-test-java-agent-log4j2' to enable it or
'--enable-test-java-agent-all' to enable all Java agents tests.

To run only this new test, use this command :

  cd tests/regression && make check TESTS="ust/java-log4j2/test_agent_log4j2_domain_log4j2"

Change-Id: Idfac151d2e523b5ac109f2dae2f182b0bc9415d8
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>