Change "Discouraged Reference" to warning

Change "Discouraged Reference" to warning

In alltests plug-in, this allows easier reuse of SWTBotStressTests
to run an individual test that is outside the alltests plug-in
(and is internal).

Change-Id: Ie570d5df176f5eb7d25c72070f296cd6d4d1b324
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/56675
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Hudson CI