Update SolrTestCaseJ4 Abstract Base Class SolrDocument comparators to static

I'd like to be able to simply compare two SolrInputDocuments in unit tests without absorbing all the overhead that comes with the JUnit test suite. Converting the prototype of the following methods to include the static modifier transforms them into functions I can use to this end:

• compareSolrDocument
• compareSolrDocumentList
• compareSolrInputDocument
• assertSolrInputFieldEquals