Investigate Test Discovery Exceptions in the DTFx CI

In trying to merge PR (https://github.com/Azure/durabletask/pull/422) , we discovered that the DTFx CI is really sensitive to adding new tests. Apparently, adding _even_ a dummy test project to the build will create a `no tests found in .dll` exception in the CI.

Currently, the most practical way of bypassing this is to re-use existing test projects, which is not sustainable in the long-run. We should investigate this and get a more robust CI config in place.