ZTS: Fix two bugs in the zpool dry run tests

[AttilaFueloep]

Motivation and Context

The output changes in #17045 broke the zpool dry run tests. This PR fixes the failing tests.

Description

Rework the tests to not depend on log output format. While here, fix a bug where "\n" was printed literally, not as a line break as intended.

How Has This Been Tested?

I've run the modified tests manually forcing test failures by editing the tests accordingly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Nice, thanks for jumping on this. I was part-way through a very similar change late last night but I got tired and went to sleep instead 💤

[AttilaFueloep]

Well, it's a matter of course, if I break things I should fix them. Sorry for the duplicate work.

[robn]

Bit hard to claim you broke it five years later :) But I appreciate the diligence!

Incidentally, I've just been playing with this as maybe a useful library function:

function capture_output {
  typeset -n out=$1
  typeset cmd=$2
  shift 2
  out=$($cmd $*)
}

typeset out
log_must capture_output out zpool status

Seems to maybe solve both the need to capture output, and to ask for a test assert on the return value. But I'm just playing, haven't thought it through, and definitely not something this PR should block on I think.

[AttilaFueloep]

I like this idea, very elegant. The question would be where to put it.

Adding a function which does what we need came to my mind as well, but I decided against it since it would just be used in four places. But that's something to think of tomorrow, it's bedtime in my timezone.

[AttilaFueloep]

There is a function log_neg_expect in logapi.shlib which roughly does the opposite of what we would need here. So it would make sense to add something like log_pos_expect, especially since I've a PR in the pipe which also checks for expected output of cli tools. Before doing that I'd prefer to have a better understanding of the inner workings of the test-runner though.

Since this PR has already two reviews (thanks!) I'd prefer not to delay merging and to follow-up later.

[tonyhutter]

Merged as:
5b0c27c ZTS: Fix zpool dry run tests output formating
029c4ae ZTS: Fix zpool dry run tests depending on output format