[441483] Rename the tests plug-in into oes.tests.junit

This is more consistent with oes.tests.junit.support and
oes.tests.swtbot, and also avoid name clash with the oes.tests feature.

Now that there are no longer name clashes, also move the plug-in's
version number to 2.0.0 like the rest.

Bug: 441483
Change-Id: Ic90bd161fee074ec17cfd97d07d1ac1ab4a95466
[441483] Fix categories names in the tests update site

Bug: 441483
Change-Id: Ic595aa5b77afa5d95b978dc7c60b91609a7bd13e
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[441483] Remove some dead code in the main POM

Bug: 441483
Change-Id: Idd6e932a045c472e0ff44e1e377ba73fa986dd4f
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>