Update unit tests

[t-lini]

check which functions and options need to be added/updated, organize files using testthat() and context() to make the tests more readable

• [x] configuration
• [x] data (add tests for base dictionary)
• [x] dictionary statistics (add tests for base dictionary, new function filterByDictionary())
• [x] dictionary statistics (add more tests for regroup() when #109 is fixed
• [x] drama head
• [x] drama tail
• [x] frequencytable (by-option)
• [x] limit figures (new option other, new function filterMentioned())
• [x] presence
• [x] tfidf
• [x] utility
• [x] utteranceStatistics
• [x] add tests for figures.R, fix #114
• [x] add tests for personnel exchange #57
• [ ] figures: add tests for rankFiguresByAppearance()?
• [ ] dictionary statistics: add tests for enrichDictionary()? (dependent on pre-trained word2vec model)
• [x] save.R: add tests after #123
• [x] update tests for base_dictionary

[t-lini]

How should the figures-dataframe look like as an input for rankFiguresByAppearance()? Maybe we should include an example in the documentation (for both rank-functions).

[t-lini]

Closing this cluttered issue in favor of #130 for 3.x