Disable overlay scrollbars when running tests to prevent crash

Disable overlay scrollbars when running tests to prevent crash

When running tests, the Eclipse launcher is not used which means
that the code that usually disables the overlay scrollbars is not
executed. The overlay scrollbars under GTK2 are unstable and can
cause crash. Specifically, a crash was frequently seen in
tmf.ui.swtbot.tests on Ubuntu.

Change-Id: I130d7fd95affa4f7eb68d2c2d1e0b0134e84ee07
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/62780
Reviewed-by: Hudson CI
Reviewed-by: Francis Giraldeau <francis.giraldeau@gmail.com>