Remove pluggable agent tests from build

[CharliePoole]

Part of Epic TestCentric/testcentric-gui#971

By including tests of pluggable agents in the engine build, we create what amounts to a cyclical dependency.

• The agents all depend upon a particular engine build and include references to assemblies from that build.
• At the same time, the engine's tests need a particular engine build in order to run

By testing pluggable engines only in their own project builds and doing so thoroughly, we break that cycle and can proceed to make improvements in one project at a time.

[CharliePoole]

:tada: This issue has been resolved in version 2.0.0-alpha8 :tada:

The release is available on:

• GitHub Release
• NuGet Package