Convert run-xunit-perf to py script (#15568)

Convert run-xunit-perf to py script (#15568)

* Convert run-xunit-perf to python script

This change merges the two run-xunit-perf scripts (.sh and .cmd) into
one unified python script and updates the pipeline job to use the
pythong script. This change also updates the linux jobs to use the new
build-tests.sh generatelayoutonly command so that we don't need to pull
down corefx from the cloud anymore. The unified python script enables us
to more easily update both linux scripting and windows scripting at the
same time so that one does not lag behind the other (such as when we add
new configurations or options like slicing). This change also turns
linux testing back on by default for PRs.