Run thread sanitizer on 5.1 test CI jobs

Motivation:

Now that SwiftPM with 5.1 supports Linux thread sanitizers, we should run this as part of the normal test pass to catch threading issues.

Modifications:

Add --sanitize=thread argument to swift test commands for any job that runs Swift >=5.1.

Result:

More bugs should be caught as soon as possible now that thread races are being monitored.