[GTest] Fix with GCC11

[fredroy]

Apparently GCC11, by default, treats the "maybe uninitialized" warning as an error.
This occurs in our version of gtest.
ref: google/googletest#3219

I did not consider updating gtest itself as it could be bothersome.
And I think the "maybe uninitialized" warning as an error itself is good
(therefore no command line option fix to consider it only as a warning)

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