Add JLD2 to test env

[christiangnrd]

For JuliaGPU/GPUArrays.jl#597

No release required, just that it be merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.93%. Comparing base (98f4618) to head (f038f34).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #507      +/-   ##
==========================================
+ Coverage   81.89%   81.93%   +0.03%     
==========================================
  Files          44       44              
  Lines        2530     2530              
==========================================
+ Hits         2072     2073       +1     
+ Misses        458      457       -1     

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