chore: rework gno.land txtar package

[gfanton]

• improve overall setup for testscript for gnoland
• add coverage support to gnoland coverage from gnoland should already be supported as it doesn't use any binary execution.

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

[codecov]

Codecov Report

Attention: Patch coverage is 70.87719% with 166 lines in your changes missing coverage. Please review.

Project coverage is 66.67%. Comparing base (9129e4e) to head (9adfc59).

Files with missing lines	Patch %	Lines
gno.land/pkg/integration/testscript_gnoland.go	69.14%	128 Missing and 38 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3086      +/-   ##
==========================================
+ Coverage   63.34%   66.67%   +3.33%     
==========================================
  Files         548      550       +2     
  Lines       78652    78699      +47     
==========================================
+ Hits        49822    52474    +2652     
+ Misses      25470    22815    -2655     
- Partials     3360     3410      +50     

Flag	Coverage Δ
contribs/gnodev	61.16% <ø> (ø)
contribs/gnofaucet	14.82% <ø> (ø)
gno.land	73.57% <69.70%> (+6.44%)	⬆️
misc/genstd	79.72% <ø> (ø)
tm2	68.67% <ø> (+6.26%)	⬆️

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