[art] Expand test coverage of secondary app images

This CL introduces a new test that verifies loading secondary app images
into a classloader that has already been initialized works as expected.

Test: ./test/testrunner/testrunner.py -b --host -t 597-app-images-same-classloader
Bug: 149098478
Change-Id: If06f4c286e090569821d77d26f7fffce50473614

Change-Id: I6cff0c8f2fd8e40546d28674c0324cb11a29bbf7