Move testutil.TB to testtb.TB

[AlekSi]

Description

That removes the import cycle in the next PR.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3022 (cafab92) into main (7f4d831) will increase coverage by 0.01%.
The diff coverage is 87.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3022      +/-   ##
==========================================
+ Coverage   76.21%   76.23%   +0.01%     
==========================================
  Files         385      385              
  Lines       21045    21045              
==========================================
+ Hits        16039    16043       +4     
+ Misses       4080     4076       -4     
  Partials      926      926              

Impacted Files	Coverage Δ
internal/util/testutil/hexdump.go	20.00% <0.00%> (ø)
integration/setup/listener.go	72.78% <66.66%> (ø)
internal/util/testutil/dump.go	45.45% <66.66%> (ø)
internal/util/testutil/equal.go	58.44% <80.00%> (ø)
integration/setup/setup.go	79.77% <83.33%> (ø)
integration/helpers.go	84.70% <100.00%> (ø)
integration/setup/client.go	70.96% <100.00%> (ø)
integration/setup/setup_compat.go	92.37% <100.00%> (ø)
integration/setup/test_helpers.go	100.00% <100.00%> (ø)
internal/util/testutil/db.go	90.47% <100.00%> (ø)
... and 4 more

... and 6 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	72.74% <82.45%> (+0.01%)	⬆️
mongodb	5.34% <54.38%> (ø)
pg	66.19% <77.19%> (ø)
shard-1	55.21% <75.43%> (ø)
shard-2	55.48% <73.68%> (+0.06%)	⬆️
shard-3	56.66% <68.42%> (-0.07%)	⬇️
sqlite	45.90% <82.45%> (+0.01%)	⬆️
unit	24.26% <61.53%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

LGTM