release-23.2: roachtest: clean up command-line flags

[RaduBerinde]

Backport 1/1 commits from #112078.

/cc @cockroachdb/release

Release justification: test-only change, keeping roachtest in sync.

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This PR is only for the last commit. The rest are #111811

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roachtest: clean up command-line flags

The code around command-line flags is pretty messy: they're defined in
many places; the name and description of a flag are far away from
the variable; the variable names look like local variables and in many
cases it's not obvious we're accessing a global.

This commit moves all flags to a separate subpackage,
roachtestflags, making all uses of global flags obvious. We also add
a bit of infrastructure to allow defining all information about a flag
right next to where the variable is declared.

We also provide a Changed() function that determines if a flag value
was changed (without the caller having to use the Command or even the
flag name).

There should be no functional changes (just some cosmetic improvements
to the flag usage texts).

Epic: none
Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
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  here).
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• 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. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[blathers-crl]

Your pull request contains more than 1000 changes. It is strongly encouraged to split big PRs into smaller chunks.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[RaduBerinde]

https://teamcity.cockroachdb.com/viewQueued.html?itemId=12684305

[RaduBerinde]

TFTR! Run looks good.