Fix xmlvalidate tests

[djjuhasz]

Fix the xmlvalidate tests so they don't fail in environments that don't have xmllint installed.

• Inject an XSD validator into the xmlvalidate activity to allow a variety of implementations
• Use a test XSD validator for tests
• Add a CLI xmlvalidate command for testing
• Add tests for the xmllint based XSD validator that only run when the "xmllint" tag is passed to the test runner

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.88%. Comparing base (da08add) to head (2ba4ae0). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
xmlvalidate/xmllint_validator.go	77.77%	5 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #33 +/- ## ========================================== - Coverage 82.98% 81.88% -1.11% ========================================== Files 15 16 +1 Lines 429 436 +7 ========================================== + Hits 356 357 +1 - Misses 50 55 +5 - Partials 23 24 +1 ```

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[djjuhasz]

@sevein okay, I'll switch to using an environment variable.