stitch: Don't run filesystem tests in parallel
stitch: Don't run filesystem tests in parallel

Because filesystem tests overwrite `util.AppFs`, we cannot run them in
parallel. Otherwise, one test may overwrite files that are needed by
another test.

kklin/kelda,quilt/quilt,tejasmanohar/quilt,LuiseV/quilt,ejj/quilt-dev,kklin/quilt,kklin/kelda,kelda/kelda,kklin/quilt,kelda/kelda,ejj/quilt-dev,LuiseV/quilt,kayousterhout/quilt,ejj/quilt-dev,NetSys/quilt,quilt/quilt,kelda/kelda,kayousterhout/quilt,LuiseV/quilt,kelda/kelda,kklin/quilt,NetSys/quilt,ejj/quilt-dev,NetSys/quilt,tejasmanohar/quilt,tejasmanohar/quilt,quilt/quilt,NetSys/quilt,tejasmanohar/quilt,LuiseV/quilt,LuiseV/quilt,kayousterhout/quilt,kklin/kelda,kklin/kelda,kayousterhout/quilt,kklin/quilt,NetSys/quilt,quilt/quilt