Improve test helpers testing

[juarezr]

This PR has the objective of improving pytest helpers testing

Changes

1. Added new helper get_env_vars_named available for pytest for further use
2. Fixed a bug in test helper ieq
3. Fixed warnings in pytest of xml and transform.regex.capture
4. Improved the docs about test helper ieq, eq_

Checklist

Use this checklist for assuring the quality of pull requests that include new code and or make changes to existing code.

• [X] Source Code rules apply:
  • [X] Includes unit tests
  • [X] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [X] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes are documented in docs/changes.rst
• [X] Versioning and history tracking rules apply:
  • [X] Using atomic commits when possible
  • [X] Commits are reversible when possible
  • [ ] There is no incomplete changes in the pull request
  • [ ] There is no accidental garbage added in source code
• [X] Testing rules apply:
  • [X] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [X] Ready to review
  • [X] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 1865084724

• 71 of 73 (97.26%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 92.099%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
petl/test/helpers.py	29	31	93.55%
<!--	Total:	71	73	97.26%	-->

Files with Coverage Reduction	New Missed Lines	%
petl/test/helpers.py	1	89.29%
<!--	Total:	1		-->

Totals
Change from base Build 1815091133:	0.1%
Covered Lines:	12555
Relevant Lines:	13632

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💛 - Coveralls