Add support for filtering out environment variables

[omajid]

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: Add support for filtering out enviornment variables in tests #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.

[omajid]

The first user of this would be anyone adapting OpenSSL 3.0. The test runner being a netcoreapp3.1 application doesn't support OpenSSL 3.0. We would like to force it to OpenSSL 1.1, but then let the actual test cases use OpenSSL 3.0 (or 1.1, whatever the SDK/Runtime under test decide to do). Part of the forcing-OpenSSL-1.1 involves setting environment variables like OPENSSL_CONF.

[omajid]

We think this is a lower priority issue that someone can address if/when they feel like it.