Add config option for fail-on-impossible-test

Test may be impossible to run, for example, because the server that's being tested does not support a particular feature.

It is not unreasonable for users to assume that, upon a successful test execution result, all tests have passed. In the case of 'impossible' tests, this isn't necessarily the case: some test might not have executed at all.

Especially in scenarios where users have configured the test run to enable a specific subset of the tests (through configuration such as enabledTests and enabledSpecifications) it may be desirable to hard fail when a test was impossible to execute. This enforces that a test run was strictly successful in all tests it was configured to execute.