Clean up "abstract" test issues

 - Deleted empty AbstractWebSocketClientTests class.

 - AbstractServletHandlerMethodTests and AbstractHttpRequestTests are
   now actually declared as abstract.

 - The following classes are not abstract but currently have an
   "Abstract" prefix and therefore get ignored by the Gradle build.
   This commit renames each of these by deleting the "Abstract" prefix.

   - AbstractFlashMapManagerTests
   - AbstractMappingContentNegotiationStrategyTests
   - AbstractSockJsServiceTests
   - AbstractWebSocketHandlerRegistrationTests