Scan classpaths for extensions only once

[bszeman9]

See details in #98

Summary

Scan classpaths for extensions only once not per test suite class.

Additional Details

#98

Checklist

Testing

(Remove this checklist and replace it with "N/A - no code changes" if this PR does not modify source code)

• I have manually verified that my code changes do the right thing.
• I have run the tests and verified that my changes do not introduce any regressions.
• I have written unit tests to verify that my code changes do the right thing and to protect my code against regressions

Documentation

(Remove this checklist and replace it with "N/A - no code changes" if this PR does not modify source code)

• I have updated the "Unreleased" section of CHANGELOG.md with a brief description of my changes.
• I have updated code comments - both GroovyDoc/JavaDoc-style comments and inline comments - where appropriate.
• I have read CONTRIBUTING.md and have followed its guidance.

[deblaci]

Well done Bence, I can't wait for try it. :)

[dpalfi]

1 if and it's a lot faster, GJ

[awittha]

Thank you!

[rh-hemartin]

Hey! Any chance of this being released?