Move test_txn13 Python test to long running test suite

The test_txn13 test creates a 4GB record to test a corner case in logging. It doesn't run reliably on some test machines. It would be good to move the test into the set of tests that only run in the "long" configuration. i.e:

python ../test/suite/run.py -v 2 --long

The current method for marking a test as long running is via the scenarios functionality in our Python test suite. It would be nice to also have a way to mark a test case as belonging in the long suite.