Migrate regression testing info pages.

[grafikrobot]

These two pages..

• https://www.boost.org/development/testing.html
• https://www.boost.org/development/running_regression_tests.html

Should be migrated into the contributor guide testing section.

[joaquintides]

As for https://www.boost.org/development/testing.html, in my opinion only the following info need be kept:

• Regression tests
• More about regression tests

As for the other two sections:

• Snapshots
• Inspection reports

The snapshots will be accessible through the regular Libraries section, selecting "master" or "develop" on the version drop-down list (not done yet). The inspection reports haven't been run since 2019.

[grafikrobot]

I think the two pages can be combine into one comprehensive page. And slimmed down as @joaquintides says.