tests: avoid false failures with LD_PRELOAD=libasan.so.2

tests: avoid false failures with LD_PRELOAD=libasan.so.2

The LD_PRELOAD checks by -fsanitize=address are overly strict:
https://groups.google.com/forum/#!topic/address-sanitizer/jEvOJgkDqQk
A workaround is to first export LD_PRELOAD=libasan.so.2
The tests below are adjusted so that workaround is not discarded.

* tests/cp/no-ctx.sh: Append to $LD_PRELOAD.
* tests/df/no-mtab-status.sh: Likewise.
* tests/df/skip-duplicates.sh: Likewise.
* tests/ls/getxattr-speedup.sh: Likewise.
* tests/rm/r-root.sh: Likewise.
* tests/cp/nfs-removal-race.sh: Likewise.  Also check that
LD_PRELOAD is effective to aid future maintainability
and avoid false failure if libasan.so.2 is not preloaded.