Split test build and run into two jobs (2nd attempt) (dotnet/coreclr#26362)

Split test build and run into two jobs (2nd attempt) (dotnet/coreclr#26362)

This change is intended to split the test job in CI runs into two
jobs - one building the tests and another sending the run to Helix.
To facilitate this, I have copied the bulk of the existing
test-job.yml file into two new files, build-test.yml and run-test.yml;
test-job.yml has become a mere wrapper invoking these two new files.

The new build-test file builds the tests, zips the output folder
and publishes it as an Azure artifact (the artifact publishing step
is no longer optional). The subsequent run-test step downloads the
test artifacts and triggers the Helix run for the test set.

The motivation for this change is to mitigate various transient
issues seen in CoreCLR CI runs - when a test execution fails in a
non-deterministic manner, before this change the developer needed to
rerun the lengthy test build step just to retry the tests. This
change should enable rerunning just the test execution step in such
case.

Thanks

Tomas

P.S. This is my 2nd attempt at the merge; Jarret had to roll back my
original Friday merge that broke CoreCLR outerloop testing.

Commit migrated from https://github.com/dotnet/coreclr/commit/0ea60e99a30f0ef9116b13546578e55a0113b1f4