qa/standalone: do not use /etc/fstab as an always-there bytes source

qa/standalone: do not use /etc/fstab as an always-there bytes source

Multiple tests use /etc/fstab when a small data file is
required as input. After all, as some comments say:
    # something that is always there

Alas - it's not always there. Not in containers.

Replacing with a newly-created temporary file filled with
random bytes.
For completeness - replacing similar references to
/etc/resolv.conf (as a source for random objects) in
the standalone tests, too.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>