Revert "Enable process isolation for unit tests."

This reverts commit 91526cd9f2.

Removing this feature primarily because it interferes with
collecting proper code coverage results. I may restore the
feature later if that can be resolved.

test/runner/lib/executor.py

@@ -624,7 +624,6 @@ def command_units(args):
 
         cmd = [
             'pytest',
-            '--boxed',
             '-r', 'a',
             '--color',
             'yes' if args.color else 'no',