[TrivialFix] Replace 'assertTrue(a in b)' with 'assertIn(a, b)'
[TrivialFix] Replace 'assertTrue(a in b)' with 'assertIn(a, b)'

Change-Id: Ib61f2f2c695f79c8eb0fa47a2d2d1bc66ccaeb7f

openstack/oslo.concurrency