Implement PyGCProtocol in test class

[Alexander-N]

For the garbage collection to work the PyGCProtocol has to be
implemented. Ideally it should not even get compiled if it doesn't since
a missing tp_traverse can produce a segfault.

Issues #323 #395

[codecov]

Codecov Report

Merging #530 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   87.55%   87.56%   +0.01%     
==========================================
  Files          65       65              
  Lines        3440     3443       +3     
==========================================
+ Hits         3012     3015       +3     
  Misses        428      428

Impacted Files	Coverage Δ
src/class/gc.rs	94.59% <0%> (+0.47%)	⬆️

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[kngwyu]

Thanks but... isn't this PR overlap #531?

[pganssle]

@kngwyu No, the two are complementary (and in fact this one needs to be merged first). PR #531 enforces at compile time that you implement PyGCProtocol for anything using the gc flag. This PR implements PyGCProtocol for the objects in the test suite that are using the gc flag.

[kngwyu]

@pganssle
I see, thanks.