releng: Do not build the testing feature when some tests are skipped

releng: Do not build the testing feature when some tests are skipped

The testing feature, as well as the "alltests" plugin, cannot be built
when -Dskip-automated-ui-tests is on, as they require all SWTBot tests.
This means the the releng-site cannot be built either. All of them
should only be run when the tests are active.

Change-Id: I9d89444818c2373239dbcd04e8749ab6e033038c
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/83830
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-by: Hudson CI