Support googletest with the CMake package name GTest

[cjmayo]

This check fails for me, (and I guess for the alternative Ubuntu libgtest-dev package:
https://packages.ubuntu.com/noble/amd64/libgtest-dev/filelist
)

According to the CMake docs the lower-case name will get checked too. Let's see...

[cjmayo]

Looks good in https://github.com/elalish/manifold/actions/runs/11419156593/job/31773433403?pr=1002
the only ones downloading from source are still clipper2 and nanobind.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.81%. Comparing base (d437097) to head (05f57d5).
Report is 127 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
- Coverage   91.84%   85.81%   -6.03%     
==========================================
  Files          37       62      +25     
  Lines        4976     9616    +4640     
  Branches        0     1049    +1049     
==========================================
+ Hits         4570     8252    +3682     
- Misses        406     1364     +958     

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[elalish]

Thanks!