Move checkLock method to TestTxnDbUtil

Referring to a method from a different unit test class is crazy, we need to move it.
When calling a method like TestDbTxnManager2.checkLock, the @ClassRule of TestDbTxnManager2 is called which can potentially screw up things, so this is kind of blocking some stuff in HIVE-27972 as well. Unit test classes should only depend on each other in the same inheritance chain.