swtbot: Delete projects after tests

This allows running the tests repeatedly

Change-Id: I289da60f0d3dc8d2539eeb5bc6f2b14e3bb35fb1
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/37752
Tested-by: Hudson CI