Add Log4j 2.x agent tests for the 'log4j' domain

Add integration tests for the new Log4j 2.x agent in Log4j 1.x compat
mode using the current 'log4j' domain, use the new configure switch
'--enable-test-java-agent-log4j2' to enable it.

To run only this new test, use this command :

  cd tests/regression && make check TESTS="ust/java-log4j2/test_agent_log4j2_domain_log4j"

Change-Id: Id780c9ee13913c91c18548f58b14cc600e77e9fa
Depends-on: lttng-ust: I7fd5f79ad58c77175714bd4198d8ff5db2e6b846
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>