More support for filtering out environment vars

[nicrowe00]

This adds more support for filtering out environment variables. OPENSSL_CONF was filtered out in a previous PR, ASPNETCORE_URLS and NUGET_PACKAGES are explicity filtered out along with it. Variables beginning with the prefixes "DOTNET_" and "CLR_" are also filtered out, with the option to allow some variables beginning with those prefixes in, like "DOTNET_ROOT".

Closes #51

[tmds]

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

Does this hacks/workarounds motivation still apply?

My review feedback: if we're filtering variables, all that gets filtered out should be printed in the runner output because it may be non-obvious to the user of the test runner that they got filtered.