Add support for filtering out environment variables

The test runner (this repository) as well as the test cases themselves are written in .NET.

Sometimes we will need to apply hacks/workarounds to get the test runner working, such as setting environment variables to control what the .NET runtime does, or what system library is loaded, or whether a library feature like invariant mode is enabled. We wouldn't generally want to pass these to the tests themselves, because we want the tests to run in a pristine environment, unaffected by hacks/workarounds applied to the test runner

We should do a few things:

• Filter out some environment variables by default, such as those beginning with DOTNET_, and CLR_.
  • Partially implemented via: https://github.com/redhat-developer/dotnet-bunny/pull/53
• Maybe: a way to pass some of those variables along, basically, selectively undo'ing the previous point.
• Let the user provide some additional environment variables to filter out via file or the command line.

Start by reviewing pull request #53 and locating the test runner's environment-handling and configuration or command-line entry points. Clarify whether default filtering, selective pass-through, and user-provided filters are all required, then add coverage showing the runner's variables stay isolated from test processes.