release-21.2: logictest: attempt to deflake "disallow full scans" test

[blathers-crl]

Backport 1/1 commits from #70278 on behalf of @yuzefovich.

/cc @cockroachdb/release

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We've seen a couple of flakes on the testing of "disallow full scans"
feature although I couldn't reproduce it manually. In an attempt to
deflake the test, this commit increases the corresponding session
variable as well as injects the stats manually instead of analyzing the
table (to have more tight control over events).

Fixes: #70012.

Release note: None

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[yuzefovich]

Although it is a testing only change, I'll hold off until 21.2.0 is out.

[michae2]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[yuzefovich]

@rytaft I'm wondering whether it is ok to merge this now given that we have seen a flake on 21.2 branch and haven't seen new flakes on master? It is testing only change.

[rytaft]

Go for it

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[yuzefovich]

TFTR!