Fix up concurrent test zpool setup and teardown

[ngie]

Summary

Set up zpools with a more unique name, stash the zpool name away in a file pointed
to by $ZPOOL_NAME_FILE (which is relative to a per-testcase generated temporary
directory), then remove the file based on $ZPOOL_NAME_FILE in the cleanup
routines.

This is a more concurrency-safe solution and will allow the testcases to be safely
executed in parallel.

MFC after: 1 week

Test Plan

kyua test passed

[ngie]

Add a sanity check to the deep setup routine as well and add a comment as to why test -n is being executed

[kevans]

LGTM, thanks! =)

[jtl]

Feel free to commit with "Approved by: jtl".

sbin/bectl/tests/bectl_test.sh

36 ↗

(On Diff #53401)

Unless I'm mistaken, there's no guarantee of uniqueness here. Instead, it seems like this makes it highly likely that there will not be a name conflict, but doesn't completely eliminate it.

It might be worth qualifying the commit message slightly to indicate this.

[ngie]

sbin/bectl/tests/bectl_test.sh

36 ↗

(On Diff #53401)

mktemp -u certainly isn't perfect, but it's considerably better than always providing a deterministic zpool name.

I added "more" to the commit message to note that this isn't perfect.