test harness: avoid possible fork bomb This fixes a regression w.r.t. the master branch, exposed by test 'parallel-tests-fork-bomb.test'. * lib/am/check.am (am--redo-logs): Detect possible infinite recursion due to a test log in $(TEST_LOGS) being the same as $(TEST_SUITE_LOG). * tests/parallel-tests-fork-bomb.test: Enhance and extend a little now that we have more explicit semantics.