Refactor / remove RunTests

[dibarbet]

See #62138 (comment)

Now that we run tests on helix for partitioning, the purpose of run tests isn't super clear. We should refactor this out to better define responsibilities, for example

1. Building test partitions for helix
2. Preparing the test payloads
3. Running integration test retry

We likely don't need much of the existing code in run tests that just calls out to dotnet test since helix is now responsible for parallelizing tests.