Add environment image library support data.

[adamsilverstein]

In https://github.com/WordPress/phpunit-test-runner/pull/134 we are adding the collection of environmental data about the GD/Imagick support of the tested environment.

We should display that data in the report once it is available, as suggested in this comment: https://github.com/WordPress/phpunit-test-runner/pull/134#issuecomment-747422071. The two locations to consider are: https://github.com/WordPress/phpunit-test-reporter/blob/master/parts/single-result.php#L50 https://github.com/WordPress/phpunit-test-reporter/blob/master/src/class-display.php#L337

[adamsilverstein]

@getsource when you have a chance, this is ready for review.