Add Tests

[sroertgen]

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

I think these functions and features should be tested:

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