Refactor test infra to work with a managed SolrClient; ditch TestHarness

This is a proposal to substantially refactor SolrTestCaseJ4 and some of its intermediate subclasses in the hierarchy.  In essence, I envision that tests should work with a SolrClient typed "solrClient" field managed by the test infrastructure. With only a few lines of code, a test should be able to pick between an instance based on EmbeddedSolrServer (lighter tests), HttpSolrClient (tests HTTP/Jetty behavior directly or indirectly), SolrCloud, and perhaps a special one for our distributed search tests. STCJ4 would refactor its methods to use the solrClient field instead of TestHarness. TestHarness would disappear as-such; bits of its existing code would migrate elsewhere, such as to manage an EmbeddedSolrServer for testing.

I think we can do a transition like this in stages and furthermore minimally affecting most tests by adding some deprecated shims. Perhaps STCJ4 should become the deprecated shim so that users can still use it during 7.x and to help us with the transition internally too. More specifically, we'd add a new superclass to STCJ4 that is the future – "SolrTestCase".

Additionally, there are a bunch of methods on SolrTestCaseJ4 that I question the design of, especially ones that return XML strings like delI (generates a delete-by-id XML string) and adoc. Perhaps that used to be a fine idea before there was a convenient SolrClient API but we've got one now and a test shouldn't be building XML unless it's trying to test exactly that.

For consulting work I once developed a JUnit4 TestRule managing a SolrClient that is declared in a test with an annotation of @ClassRule. I had a variation for SolrCloud and EmbeddedSolrServer that was easy for a test to choose. Since TestRule is an interface, I was able to make a special delegating SolrClient subclass that implements TestRule. This isn't essential but makes use of it easier since otherwise you'd be forced to call something like getSolrClient(). We could go the TestRule route here, which I prefer (with or without having it subclass SolrClient), or we could alternatively do TestCase subclassing to manage the lifecycle.

Initially I'm just looking for agreement and refinement of the approach. After that, sub-tasks ought to be added. I won't have time to work on this for some time.