Update WindowsAppSDK-RunHelixTests-Job.yml

[alexlamtest]

Migrated MRT's MrmBaseUnitTests, MrmUnitTest, and MrtCoreUnpackagedTests test suites from Helix to the pipeline test environment, leveraging the recently introduced .testdef mechanism for onboarding them. There is then no longer a need for WindowsAppSDK-RunHelixTests-job.yml, hence it is deleted in this PR.

Those 3 test suites were also being directly run on the current build job agent via the vstest task. That looks redundant now in the general case, because in the pipeline environment all TAEF tests are being run on many OS images, instead of just one version of Server. Hence, WindowsAppSDK-BuildAndTestMRT-Steps.yml is updated to stop running the 3 MRT test suites, unless the code is called from the CI pipeline, because in this specific flow we currently don't run any tests later in the 1ES hosted pipeline environment.

This PR not only allows us to drop dependency on Helix (whose support is dwindling) from the Foundation repo, but also makes the pipeline more efficient, e.g., in a sample pipeline run in the past: https://dev.azure.com/microsoft/ProjectReunion/_build/results?buildId=64221252&view=results, the "TestInHelix Release_x64" job that runs the MRT tests took 25m 23s.
In an early pipeline run of this PR: https://dev.azure.com/microsoft/ProjectReunion/_build/results?buildId=66122428&view=results, the "PipelineTestsX64 rs_prerelease_x64fre" that runs the same MRT suites, plus a couple of other pre-existing test suites there, took 12m 59s, i.e., a speed up of ~50%.

CR: Michael, Will, Robert

How built:

• Changes built fine when triggering the TransportPackage-Foundation-PR pipeline manually
• PR validation

How Tested:

• In a test pipeline run of this PR: https://dev.azure.com/microsoft/ProjectReunion/_build/results?buildId=66131963&view=results, MRT's MrmBaseUnitTests.dll, MrmUnitTest.dll, and MrtCoreUnpackagedTests.dll produced results below, respectively, as expected in the "PipelineTestsX64 rs_prerelease_x64fre" job:
  Summary: Total=268, Passed=268, Failed=0, Blocked=0, Not Run=0, Skipped=0
  Summary: Total=21, Passed=21, Failed=0, Blocked=0, Not Run=0, Skipped=0
  Summary: Total=27, Passed=27, Failed=0, Blocked=0, Not Run=0, Skipped=0

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[alexlamtest]

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[codendone]