improve `degree` and more test fixes

[codecov[bot]]

Codecov Report

Merging #268 (b999d31) into master (7397577) will increase coverage by 0.37%. The diff coverage is 98.75%.

@@            Coverage Diff             @@
##           master     #268      +/-   ##
==========================================
+ Coverage   81.75%   82.12%   +0.37%     
==========================================
  Files          17       19       +2     
  Lines        1869     1925      +56     
==========================================
+ Hits         1528     1581      +53     
- Misses        341      344       +3     

Impacted Files	Coverage Δ
src/layers/conv.jl	79.18% <83.33%> (-0.04%)	:arrow_down:
src/GNNGraphs/generate.jl	100.00% <100.00%> (ø)
src/GNNGraphs/gnngraph.jl	56.81% <100.00%> (+1.34%)	:arrow_up:
src/GNNGraphs/query.jl	96.13% <100.00%> (+1.51%)	:arrow_up:

... and 4 files with indirect coverage changes

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