Enhance object-cache.php drop-in placement logic to support updating to a newer version of the file

[felixarntz]

Summary

See #1042 (comment): Given that PR contains changes to the object-cache.php drop-in which require updating it in order for the underlying Server-Timing logic to still be loaded correctly, the logic to place that file needs to support updating an older version of the file to its latest version that comes with Performance Lab.

So far, the logic was naively checking for purely the existence of the drop-in via its version constant, without checking whether that version is the latest version though.

Relevant technical choices

• The logic bails if the PL drop-in is already present and in the latest version (or newer, though that's unlikely).
• Otherwise, it enters the logic as before. The new change there is that, if there is an existing drop-in, it may be an older version of the Performance Lab one.
  • If the version constant is not set, for sure it's not the PL drop-in.
  • Otherwise, it likely is the PL drop-in, though just to make sure this is verified by checking the file contents.
  • If it is indeed an older version of the PL drop-in, that file is deleted, so that afterwards the previously existing logic can copy over the new drop-in.
• A new constant PERFLAB_OBJECT_CACHE_DROPIN_LATEST_VERSION contains always the latest version of the drop-in. For that, we need a separate constant as we can't access the PERFLAB_OBJECT_CACHE_DROPIN_VERSION constant defined in the plugin's own object-cache.copy.php file, as that file is never actually loaded.
• A filter perflab_object_cache_dropin_version is introduced, purely to facilitate unit testing related to the PERFLAB_OBJECT_CACHE_DROPIN_VERSION constant. This filter is needed because constants can't be "un-defined", so code relying on constants is otherwise untestable.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[westonruter]

LGTM, but with some test assertion hardening.

[felixarntz]

PR is updated to address the remaining feedback as well as to cater for the object-cache.copy.php updates merged via #1042.