XUnitHelper fails as soon as the test run for any single assembly fails, not running tests for other assemblies.

The `XUnitHelper` will fail as soon as a signle test failed, not continuing to run the rest of the tests, in other assemblies.

It is useful to see all test results, regardless of whether any tests fail. I found it especially useful for generating test reports for a CI server.

I've changed the `xUnit` function to call xunit runner for every assembly, and in the end, if any of the calls failed, to `failwith` and report a list of offending assemblies. It works fine in my environment.