Fully replace Orbit's JUnit 5 by upstream Maven one

[mickaelistria]

In order to simplify consumption of upstream libs and publication to Maven, we prefer using upstream Maven bundles in favor of Orbit's ones. Currently both Orbit's and Maven's JUnit 5 artifacts are in the .target file. We should remove Orbit's variant as soon as possible. This require to update some code in JDT and PDE to allow working with different artifacts.

• Replace JUnit bundle req by Import-Package eclipse.platform.releng#68
• Replace req on JUnit 5 by Import-Package eclipse.platform.ui#192
• Add JUnit 5 from Maven Central eclipse-jdt/eclipse.jdt#25
  Need immediate fix:
• org.eclipse.e4.ui.tests.css.core not running with Tycho eclipse.platform.ui#204
  Next steps:
• Use JUnit 5 from Maven Central: change in Bundle-SymbolicName eclipse-jdt/eclipse.jdt.ui#138
• Remove useless bundle requirements eclipse-jdt/eclipse.jdt.ui#139
• Use JUnit from Maven Central eclipse-pde/eclipse.pde#199
  Then when all plugins are supposed to work with junit- (Maven Central) bundles,
• Remove Orbit bundles as we use Central ones eclipse-jdt/eclipse.jdt#27
  Other fixes
• Remove Orbit bundles as we use Central ones (cont'd) eclipse-jdt/eclipse.jdt#28
• Adapt to JUnit from Maven Central eclipse.platform#46
• Remove Orbit's JUnit 5 #402

[iloveeclipse]

@mickaelistria: could you please check if #409 caused by the changes here, and also eclipse-equinox/equinox#81?

[mickaelistria]

Done. Let's reopen if next I-Build show other JUnit related issues.