Improve NUnit xml report

[gnuechtel]

Fix for #1179

[nohwnd]

@cgnuechtel fails this in 9 places (you should be able to see that in the details as well I hope). I guess you are using . navigation from array to child item property which does not work in PowerShell 2. You need Select-Object -ExpandProperty Node most likely, but I haven't looked at the code.

 [-] should contain all scenarios of feature 1 with correct names and test results 13ms
2018-12-20T19:43:23.4716250Z       RuntimeException: Property 'Node' cannot be found on this object. Make sure that it exists.
2018-12-20T19:43:23.4716250Z       at line: 299 in C:\agent\_work\3\s\Functions\Gherkin.Tests.ps1

[nohwnd]

@cgnuechtel LGTM. @fourpastmidnight any objections to merging this?

[gnuechtel]

I would like to suggest more small but important improvements with the last two commits.

1. I think the position of the invocation of $Pester.EnterTestGroup was not correct, since it produced nested elements in case of error
2. I also think, that the file name should be replaced by the feature name in the test report (in any case of error the file name would occur in the stack trace)

[nohwnd]

@cgnuechtel nobody objects, so merging.