Overhaul GraphQL test suite

[zshipko]

• Adds a query type to the graphql tests:
• Makes writing queries in tests easier (see https://github.com/mirage/irmin/pull/1750/files?diff=split&w=0#diff-3131055e1ed5ebb5e09f20fe0b53836c27ac4205cde1805309c678198c2c499dR98)
• Allows for response to be parsed (almost) automatically

[codecov-commenter]

Codecov Report

Merging #1750 (f7c5eff) into main (e54478b) will increase coverage by 0.35%.
The diff coverage is 100.00%.

❗ Current head f7c5eff differs from pull request most recent head 4a972ad. Consider uploading reports for the commit 4a972ad to get more accurate results

@@            Coverage Diff             @@
##             main    #1750      +/-   ##
==========================================
+ Coverage   64.63%   64.98%   +0.35%     
==========================================
  Files         116      116              
  Lines       14159    14164       +5     
==========================================
+ Hits         9152     9205      +53     
+ Misses       5007     4959      -48     

Impacted Files	Coverage Δ
src/irmin-graphql/server.ml	59.95% <100.00%> (+10.42%)	⬆️
src/irmin-test/store.ml	94.93% <0.00%> (-0.06%)	⬇️
src/irmin/tree.ml	81.42% <0.00%> (+0.14%)	⬆️
src/irmin/store.ml	64.96% <0.00%> (+0.29%)	⬆️
src/irmin/commit.ml	63.98% <0.00%> (+0.32%)	⬆️
src/irmin/import.ml	73.91% <0.00%> (+4.34%)	⬆️

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