Do *not* disable indexer for all UsageTest subclasses, the tests depend
on running indexer and wait forever waiting for the disabled indexer.

Change-Id: I5335652452cc143900fcccc100d554a608a7ba12
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/182745
Tested-by: PDE Bot <pde-bot@eclipse.org>