discard test-fragment packaging

[laeubi]

Adding more stages to the test story has shown that the test-fragment packaging is too strict and does not scale well (e.g. has hard dependency to additional.bundles feature that should be generalized in #53) as this is not released/used in the wild we better discard this now before there are consumers of this feature.

The following should replace this feature:

• Support to supply additonal requirements through P2MetadataProvider #53 replace the support for additional-bundles
• Bug 572602 - Tycho should support classpath-container #37 replace the support for loading additional classes from test-classpath
• dev.properties already cover the case of test-classes to be loaded from test-source folders