Use LastIndexOf instead of IndexOf in LicenseHeaderTest

[quinmars]

My Rx git repository happens to be in the directory Rx.NET. So the resulting global source path is:

C:\Users\peter\Documents\src\Rx.NET\Rx.NET\Source.

Unfortunately, that makes the license header test fail, because in the test, the second occurance of Rx.NET is searched for but the first one is found.

The PR fix this by using LastIndexOf, addditionally the test will now fail, if there is no occurance at all instead of silently passing.

[danielcweber]

Thx. I was running into the exact same problem. I was wondering whether we can avoid the IndexOf altogether and just ...... back our way to the right directory. It might not work on all environments though (dotnet/net/local/CI/etc). Would you like to explore this?

[quinmars]

I think there cannot be a compile time constant for the source directory, at least it would not improve portability. The biggest problem I can imagine is that the tests are not excuted on the build computer but on a different device, e.g. a mobile device or different server. Then the test will fail. I haven't found a way to skip a test in xunit from code. That would probably be the best.

What I can offer is a very small improvement on the path creation.