scripts: testedtlib.py: Generate SystemExit on test failures

The devicetree check in check_compliance.py in ci-tools expects the
dtlib/edtlib test suites to exit with sys.exit() (which raises
SystemExit) on test failures, and interprets Exception as an internal
error in the test suite.

testedtlib.py accidentally raised Exception on test failures, making
check_compliance.py error out and skipping the rest of the tests when
there were failures. Fix it.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>