[Bug]: Output mangled when Terminal Logger used in combination with dotnet test

[JanKrivanek]

Reported by @baronfel

Context

Terminal Logger and tests running target are both trying to use the output stream in output rewinding/overwriting way - resulting in mangled output

Root cause: microsoft/vstest#1503

Discussed quick fixes for now:

• Disable TerminalLogger if running from the dotnet test (with adding low-pri message about this - especially for cases where the Terminal Logger was explicitly opted in)
• Flip TerminalLogger of once detecting that the test target is starting

Other possible solutions:

• Adding a quiet mode to Terminal Logger (to avoid any output except the warns/errors)