testimage: print results for interrupted runs

When a run is ended by overall timeout, print the already executed
testcases, to provide some hints which testcase might made the
test suite reach global timeout.
Nonetheless make the testrun exit with an error

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>