Clean up test classes

[bonaparten]

Through the new versions of the ohsome API some past tests are out-of-date.
A general review of all tests and their functions is needed, e.g. there are some duplicated tests elementsGeomUsingOneTagTest() and elementsGeomUsingMultipleTagsTest().
Where the names of test methods don't help to understand the function of the tests, comments should be added, e.g. explaining what it is meant by "general resource" and "specific parameter" getGeneralResourceWithSpecificParameterTest()