xfstests example: Use export in local.config and remove comment (#811)

xfstests example: Use export in local.config and remove comment (#811)

ix passthrough mount helper for running xfstests

* The mount helper does not see the env vars exported by xfstests
* Use the NFS style format TEST_DEV=source:/$TEST_SOURCE to communicate
  the sourse path to the mount helper without confusing xfstest
* Also recognise when source= is provided in mount options
* Support -o remount (e.g. for test generic/306)

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Co-authored-by: Bernd Schubert <bschubert@ddn.com>
Co-authored-by: Amir Goldstein <amir73il@gmail.com>
Co-authored-by: Nikolaus Rath <Nikolaus@rath.org>