Add Tests

[sroertgen]

For a while tests for new features have not been added.

I think these functions and features should be tested:

• [x] replaceFilePathInUrl
• [x] appearance of language selector when vocab has multiple languages
• [x] absent of language selector if vocab has just one language
• [x] absent of "expand" / "collapse" button when no hierarchy in vocab
• [x] Collection component is rendered
• [x] search feature is working
• [x] missing prefLabel in a language shows "no label provided"
• [x] all provided labels are shown on a concepts page (skos:prefLabel, skos:altLabel, skos:hiddenLabel)
• [x] test docker build
• [x] docker latest tag is assgined instead of just master (but master as well)
• [x] cache node_modules when running tests
• [x] test built of slash and hash uris
• [x] concept tree is shown when switching languages being on a concept
• [x] concept tree is shown when switching languages being on a collection
• [x] error message is shown when data-folder is empty