write tests for LabelTopics

[TommyJones]

1. Put them in tests/testthat/test-topic_modeling_utilities.R
2. Try to write a test for every argument or contingency the code anticipates

[TommyJones]

see #42, #43, #44, #45, #46, #47, #48, #49, #50, #51, #52, #53, #54, #55, #56, #57, #58, #59, #60, #61, #62, #63, #64

[TommyJones]

closed the wrong issue before. Oops!