Fix unnecessary recompilation in gtest Makefile

[harrylin98]

This PR fixes the issue where changes to src files did not trigger a rebuild of
libvalkey.a when rerunning make test-gtest.

It also resolves the problem where unchanged .cpp test files were unnecessarily recompiled
during make test-gtest.

Now:

If nothing changed:

• Nothing rebuilds, no relinking occurs.

If only C++ test files changed:

• Both libvalkey.a and C++ test files recompile (because release.o changes)
• Test executable relinks

If only src code changed:

• Only libvalkey.a rebuilds
• C++ test files do not recompile
• Test executable relinks

If both changed:

• Both libvalkey.a and C++ test files rebuild
• Test executable relinks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.19%. Comparing base (81bd5bd) to head (cbc5dbc).
⚠️ Report is 1 commits behind head on feature-gtest.

Additional details and impacted files

@@                Coverage Diff                @@
##           feature-gtest    #3063      +/-   ##
=================================================
- Coverage          74.33%   74.19%   -0.14%     
=================================================
  Files                129      129              
  Lines              70974    70974              
=================================================
- Hits               52756    52658      -98     
- Misses             18218    18316      +98     

see 17 files with indirect coverage changes

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[JimB123]

Looks ok. Not sure about some of the makefile intricacies, but given that this is for test build, risk is low. Further refinements are easy to make.

I expect additional changes/refinements as this is being prepared in the "feature-gtest" branch.

[madolson]

Sure