Add support for running list of tests in manifest

[bstpierre]

Adding "test_list": ["test1", "test2"] to the keys in the manifest declares a list of STF test targets.

Running p4app test myapp.p4app will run all of the tests in the list, and report a summary at the end, in the form:

TEST SUMMARY
test1: PASS
test2: FAIL

[theojepsen]

@bstpierre can you edit the README, documenting how this new manifest option should be used?