Metastore: Sporadic unit test failures

For instance: https://builds.apache.org/job/Hive-trunk-h0.21/1792/testReport/org.apache.hadoop.hive.metastore/

Found the following issues:

testListener: Assumes that a certain tmp database hasn't been created yet, but doesn't enforce it

testSynchronized: Assumes that there's only one database, but doesn't enforce the fact

testDatabaseLocation: Fails if the user running the tests is root and doesn't clean up after itself.