Fixes #249: Fix IDE diagnostics test framework to use DiagnosticAnalyzerService to compute diagnostics rather than DiagnosticAnalyzerDriver.

This change serves multiple purposes:
1) Makes our test framework consistent with the IDE scenario in fetching diagnostics. We have missed out on multiple regressions where the core IDE DiagnosticAnalyzerDriver does the right thing and hence the tests pass, but the DiagnosticService in IDE does additional filtering and certain diagnostics are not reported in VS. This change should avoid such bugs.
2) Enable testing for diagnostics V2 engine work that JohnHamby is doing to merge IDE and compiler analyzer drivers. For this engine, there is no IDE driver and hence existing IDE diagnostics tests can't be run. This change should enable running IDE diagnostics on V2 engine.
3) Move a whole bunch of of test only code in DiagnosticAnalyzerService_Test.cs into the test layer.