Better handle filesystems mounted with -o noexec

ftruncate/11, open/20, and truncate/11 copy sleep(1) to a temporary file on the
target filesystem, execute the binary, then expect a result. This doesn't work
with scenarios where the target binary cannot be executed, e.g. the filesystem
was mounted with -o noexec.

Ensure the filesystem hasn't been mounted with -o noexec for the testcases
before executing them.

Differential Revision: https://reviews.freebsd.org/D5622
MFC after: 1 week
Reviewed by: markj
Sponsored by: EMC / Isilon Storage Division