Merge assertj-core-java17 into assertj-core

[joel-costigliola]

Merge assertj-core-java17 into assertj-core

[joel-costigliola]

Should we close this issue as duplicate ?

[scordio]

During the work of #3442 and #3446, I realized we should have a slightly different testing strategy (see #3442 (comment)).

Wherever possible, we should favor extra-modular testing. while in-module testing should be used only when we want to test package-private features.

This means we should have:

• assertj-tests/assertj-integration-tests/assertj-core-module-path for assertj-core for extra-modular testing
• assertj-tests/assertj-integration-tests/assertj-guava-module-path for assertj-guava extra-modular testing
• assertj-core/src/test restricted to in-module testing

Currently, there are no package-private relevant tests for assertj-guava so we can avoid having assertj-guava/src/test.

I'm not sure if I'll be able to slice all these changes in smaller steps so I'm going to assign this one to me.

[scordio]

Superseded by #3446.