Port and sync unit testing approach from fjson to tjson

[rumyantseva]

Description

This PR closes #899.

This PR contains the simplest test cases migrated from fjson.
More complicated cases (e.g. invalid schema) should be covered separately.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• (for maintainers only) I set Reviewers, Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #935 (7921f08) into main (c0fa49b) will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #935      +/-   ##
==========================================
+ Coverage   62.40%   62.57%   +0.16%     
==========================================
  Files         225      225              
  Lines       10444    10444              
==========================================
+ Hits         6518     6535      +17     
+ Misses       3155     3138      -17     
  Partials      771      771              

Impacted Files	Coverage Δ
internal/handlers/pg/pgdb/databases.go	54.66% <0.00%> (-5.34%)	⬇️
internal/handlers/pg/pgdb/collections.go	54.86% <0.00%> (-5.31%)	⬇️
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/handlers/pg/msg_dbstats.go	72.09% <0.00%> (+6.97%)	⬆️
internal/tjson/tjson.go	66.66% <0.00%> (+7.80%)	⬆️
internal/tjson/int64.go	59.09% <0.00%> (+13.63%)	⬆️
internal/tjson/double.go	59.09% <0.00%> (+13.63%)	⬆️
internal/tjson/bool.go	45.45% <0.00%> (+45.45%)	⬆️

Flag	Coverage Δ
integration	57.28% <ø> (ø)
mongodb	18.80% <ø> (-0.04%)	⬇️
pg	62.41% <ø> (ø)
tigris	18.31% <ø> (-0.03%)	⬇️
unit	24.50% <ø> (+0.58%)	⬆️

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