Add a unit test for JavaAgent preconditions and attribute wiring.

Testing Done:
Failed after test add, fixed after JavaAgent fix:
==
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/targets --test-pytest-options="-v -kJavaAgentTest"
...
20:42:36 00:00   [test]
20:42:36 00:00     [pytest]
20:42:36 00:00       [run]
                     ============== test session starts ===============
                     platform linux2 -- Python 2.6.9 -- py-1.4.22 -- pytest-2.6.0 -- /usr/bin/python2.6
                     plugins: cov, timeout
                     collected 40 items

                     tests/python/pants_test/targets/test_java_agent.py@55::JavaAgentTest::test_can_redefine PASSED
                     tests/python/pants_test/targets/test_java_agent.py@66::JavaAgentTest::test_can_retransform PASSED
                     tests/python/pants_test/targets/test_java_agent.py@77::JavaAgentTest::test_can_set_native_method_prefix PASSED
                     tests/python/pants_test/targets/test_java_agent.py@39::JavaAgentTest::test_defaults PASSED
                     tests/python/pants_test/targets/test_java_agent.py@35::JavaAgentTest::test_minimal PASSED
                     tests/python/pants_test/targets/test_java_agent.py@31::JavaAgentTest::test_required PASSED

                     ==== 34 tests deselected by '-kJavaAgentTest' ====
                     ==== 6 passed, 34 deselected in 0.38 seconds =====

20:42:38 00:02     [junit]
20:42:38 00:02     [specs]
               SUCCESS

Bugs closed: 403

Reviewed at https://rbcommons.com/s/twitter/r/770/