Partially initialized SnapshotHFileCleaner leads to NPE during TestHFileArchiving

TestHFileArchiving#testCleaningRace creates HFileCleaner instance within the test.
When SnapshotHFileCleaner.init() is called, there is no master parameter passed in params.

When the chore runs the cleaner during the test, NPE comes out of this line in getDeletableFiles():

      return cache.getUnreferencedFiles(files, master.getSnapshotManager());

since master is null.

We should either check for the null master or, pass master instance properly when constructing the cleaner instance.