Fix Azure coverage and merge coverage over pipeline jobs

[johlju]

Pull Request

Pull Request (PR) description

Added

• Pipeline updated to support merging code coverage between operating system pipeline jobs.

  Fixed

• Fix uploading to Azure Code Coverage.
• _Merge_CodeCoverageFiles
  • Fixed so the file that is outputted is in UTF-8 (without BOM) to support Codecov.io.
  • The task now only searches for the file pattern inside the ./output/testResults folder.
  • The merge process is not attempted if CodeCoverageThreshold is set to 0.
  • Updated so that build.yaml now have a key CodeCoverage which have to settings CodeCoverageMergedOutputFile and CodeCoverageFilePattern.
• _Convert_PesterCoverage
  • The backup file now have the extension .bak (instead of .bak.xml) so that is not mistakenly used by a task _Merge_CodeCoverageFiles.
  • Some code cleanup.

Task list

• [x] The PR represents a single logical change. i.e. Cosmetic updates should go in different PRs.
• [x] Added an entry under the Unreleased section of in the CHANGELOG.md as per format.
• [x] Local clean build passes without issue or fail tests (build.ps1 -ResolveDependency).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [x] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

---

This change is 

[codecov[bot]]

Codecov Report

Merging #267 (08bb1ff) into master (fc7148a) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #267   +/-   ##
=====================================
  Coverage      13%    13%           
=====================================
  Files          24     24           
  Lines         431    431           
=====================================
  Hits           58     58           
  Misses        373    373           

[johlju]

@gaelcolas this should be ready for review now. The pipeline should pass now.