fix _strip_common_prefix and add assertions
fix _strip_common_prefix and add assertions

lstrip does not do what I thought it did. This change makes
_strip_common_prefix behave properly, and in the absence of a test suite
for helper functions, adds assertions to make sure it works correctly.

aweisberg/cassandra-dtest,mambocab/cassandra-dtest,riptano/cassandra-dtest,iamaleksey/cassandra-dtest,carlyeks/cassandra-dtest,aweisberg/cassandra-dtest,iamaleksey/cassandra-dtest,krummas/cassandra-dtest,bdeggleston/cassandra-dtest,spodkowinski/cassandra-dtest,snazy/cassandra-dtest,spodkowinski/cassandra-dtest,carlyeks/cassandra-dtest,snazy/cassandra-dtest,thobbs/cassandra-dtest,blerer/cassandra-dtest,blerer/cassandra-dtest,beobal/cassandra-dtest,pauloricardomg/cassandra-dtest,stef1927/cassandra-dtest,bdeggleston/cassandra-dtest,riptano/cassandra-dtest,krummas/cassandra-dtest,beobal/cassandra-dtest,pauloricardomg/cassandra-dtest,pcmanus/cassandra-dtest,thobbs/cassandra-dtest,stef1927/cassandra-dtest,mambocab/cassandra-dtest