tests: avoid spurious parallel failure due to temporary disk full

tests: avoid spurious parallel failure due to temporary disk full

Running the new fiemap-empty test uses 600MB of disk space via
fallocate, and in so doing caused failure in unrelated tests that
were running in parallel on a small file system.  Rather than
simply running fallocate (which allocates the space, inducing
disk full when it fails), skip the test if there is less than
800MB of free space, as computed via stat and awk.
* tests/init.cfg (require_file_system_bytes_free_): New function.
* tests/cp/fiemap-empty: Use it.