NTB: ntb_test: Safely use paths with whitespace

If some of variables like LOC/REM or LOCAL_*/REMOTE_* got
whitespaces, the script may fail with syntax error.

Fixes: a9c59ef7 ("ntb_test: Add a selftest script for the NTB subsystem")
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>