feat: implement gno test -cover

[harry-hov]

Coverage support in gno test

Fixes #1121

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (89428c5) 47.84% compared to head (7a498c6) 47.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1191      +/-   ##
==========================================
- Coverage   47.84%   47.31%   -0.53%     
==========================================
  Files         369      367       -2     
  Lines       62764    61503    -1261     
==========================================
- Hits        30028    29102     -926     
+ Misses      30308    30036     -272     
+ Partials     2428     2365      -63     

Files	Coverage Δ
gnovm/cmd/gno/test.go	24.66% <ø> (-0.06%)	⬇️
gnovm/pkg/gnolang/preprocess.go	31.18% <ø> (+0.01%)	⬆️
gnovm/tests/imports.go	77.63% <ø> (-0.13%)	⬇️

... and 14 files with indirect coverage changes

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