[TESTBUG] Fix G1 redefineClasses tests and a memory leak

My colleague Ian Rogers found a few JNI memory leaks in this test. Several calls to JNIEnv::Get*() should be paired with calls to JNIEnv::Release*().

During review, we found that several affected tests were not properly configured, so makeRedefinition0() never runs. These tests will be fixed in the same change.