zpool_trim tests: throttle trim process

[gmelikov]

Otherwise trim may finish before progress checks.

It is a prerequisite for running tests in Github VMs.

Signed-off-by: George Melikov [email protected]

Motivation and Context

Fix false negative in tests.

Description

Race condition reproduces easily in github actions' VMs, example: https://github.com/gmelikov/zfs/runs/1504242726?check_suite_focus=true

Fixed run: https://github.com/gmelikov/zfs/runs/1507040525?check_suite_focus=true

Manual commands run gives me same results - trim finishes instantly if pool was empty.

I've done the easiest method - just rewrite some blocks a count of times after race can't be reproduced.

How Has This Been Tested?

Runs in CI, please see links in description.

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.