Don't delete freebsd.run in distclean

[ghost]

Motivation and Context

Description

Add a comment so the file is not empty.

The comment can be removed when FreeBSD-specific tests are added.

How Has This Been Tested?

gmake distclean

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)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux 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.

[kithrup]

LGTM :).

[codecov-io]

Codecov Report

Merging #10206 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10206      +/-   ##
==========================================
- Coverage   79.33%   79.31%   -0.03%     
==========================================
  Files         388      388              
  Lines      123392   123392              
==========================================
- Hits        97898    97867      -31     
- Misses      25494    25525      +31     

Flag	Coverage Δ
#kernel	79.96% <ø> (+0.17%)	⬆️
#user	65.58% <ø> (+0.62%)	⬆️

Impacted Files	Coverage Δ
cmd/zvol_id/zvol_id_main.c	76.31% <0.00%> (-5.27%)	⬇️
cmd/ztest/ztest.c	76.13% <0.00%> (-4.23%)	⬇️
module/lua/lmem.c	83.33% <0.00%> (-4.17%)	⬇️
module/icp/api/kcf_mac.c	38.28% <0.00%> (-2.29%)	⬇️
module/zfs/vdev_queue.c	95.10% <0.00%> (-1.23%)	⬇️
module/zfs/zil.c	91.82% <0.00%> (-1.22%)	⬇️
module/os/linux/zfs/zpl_xattr.c	82.83% <0.00%> (-0.85%)	⬇️
module/icp/api/kcf_cipher.c	15.76% <0.00%> (-0.83%)	⬇️
module/zfs/vdev_label.c	93.87% <0.00%> (-0.69%)	⬇️
cmd/zed/agents/zfs_mod.c	77.55% <0.00%> (-0.67%)	⬇️
... and 43 more

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