MockFile.Delete does not throw exception when file does not exist

[borismod]

Unit test to reproduce:

var fs = new MockFileSystem();
fs.File.Delete(@"c:\someFilePath.txt");

The above code does not throw any exception, while the real File.Delete from System IO will throw

[jpreese]

For clarity, File.Delete from System.IO will only throw if the directory does not exist. If it tries to delete a file that does not exist, it works just fine. The former throws a DirectoryNotFoundException.

We should implement this for parity with System.IO

[jpreese]

Additional clarification.. it looks like this will break some old tests as well. We'll need to add a directory in the failing tests.

[peter-sereda]

Hello @jpreese, @fgreinacher . I have tried to fix the issue in my repository fork. Could you please check the result and tell me if something needs to be corrected?
peter-sereda#1

[fgreinacher]

Hey @peter-sereda - thanks for taking care of this. Please open a PR against this repository so that we get test results.

[peter-sereda]

@fgreinacher , done.
#387

[borismod]

@peter-sereda @fgreinacher @jpreese thanks a lot guys for your contribution. Really appreciated!