chore(core-test-framework): resolve .env and app.json file path from package name

[sebastijankuzner]

Summary

Use file path resolving based on package name rather than using relative structure used in packages/. Fixes problem which occur on booting sandbox.app when using published @arkecosystem/core-test-framework package.

Checklist

• Ready to be merged

[codecov]

Codecov Report

Merging #4006 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #4006   +/-   ##
========================================
  Coverage    98.49%   98.49%           
========================================
  Files          631      631           
  Lines        14583    14583           
  Branches      1732     1732           
========================================
  Hits         14363    14363           
  Misses         129      129           
  Partials        91       91           

Flag	Coverage Δ
#functional	6.80% <100.00%> (ø)
#integration	9.88% <50.00%> (ø)
#unit	96.66% <100.00%> (ø)

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Impacted Files	Coverage Δ
...ges/core-test-framework/src/app/generators/core.ts	96.66% <100.00%> (ø)

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