Improve Linux arm32/arm64 corefx test runs.

Improve Linux arm32/arm64 corefx test runs.

Add proper parallelism.

Prevent per-test output during parallel runs from interleaving
by saving output per process and displaying it after the
process has completed.

Improve summary test/failure output at end of log.

Change Linux arm32/arm64 corefx jobs to run tests in parallel.

Change Linux/arm64 corefx scheduled jobs to be daily, not weekly,
since they are very fast when run in parallel (<20 minutes).

Commit migrated from https://github.com/dotnet/coreclr/commit/07b8ea2ca94fc519c68f211e1494ebc7ccd588f1