[cache_unittest] Add direct implementation testing on ordering, pruning (backport #678)

[mergify]

Towards #676
This changes nothing pertaining to the implementation. It only adds testing.
Some of it is implementation-specific, but towards the ends of performance checks.

From #676 (comment)

1. It needs to hold an arbitrary number of entries, because we don't know the rate at which we will get data.
2. The items needs to be in sorted order by timestamp, from the newest entry to the oldest.
3. New items need to be inserted into arbitrary positions, as timestamps may come in out-of-order.
4. Duplicates shouldn't be allowed.
5. Old items need to be pruned out.

To answer them:

1. There appears to be no limit on size. Not sure how to test this in a bounded fashion.
2. This was not tested. Now explicitly tested via ImplSortedDescendingUniqueEntries
3. This was not tested. Now explicitly tested via ImplSortedDescendingUniqueEntries
4. This was already tested, but is also explicitly tested via ImplSortedDescendingUniqueEntries
5. This was not tested (I believe). Now explicitly tested via ImplSortedDescendingUniqueEntries

   ---

   This is an automatic backport of pull request [cache_unittest] Add direct implementation testing on ordering, pruning #678 done by Mergify.

[mergify]

Cherry-pick of a17a2bf has failed:

On branch mergify/bp/iron/pr-678
Your branch is up to date with 'origin/iron'.

You are currently cherry-picking commit a17a2bf8.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tf2/src/cache.cpp
	modified:   tf2/test/cache_unittest.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tf2/include/tf2/time_cache.h

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ahcorde]

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