Add utility method to close secondary shells after tests

Add utility method to close secondary shells after tests

When running SWTBot tests, sometimes when one test fails and leave a
shell lingering, it makes all the subsequent tests fail. This change
introduces a utility method to close all the secondary shells
(non-main shell) so that subsequent tests have a better chance of
completing successfully. This will help getting a more representative
account of test failures when things go wrong.

Change-Id: I4fe48c13de8588e9ddb21ae64d2c3b6a9c04b4ae
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/81303
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-by: Hudson CI