Remove testInstance parameter from TestInstancePostProcessor

[sbrannen]

Overview

Due to recent refactorings that made the test instance available in the ExtensionContext, a TestInstancePostProcessor now has two ways to access the test instance object in the postProcessTestInstance() method, thereby making the Object testInstance parameter superfluous.

Related Issues

• Allow @BeforeAll and @AfterAll methods to be non-static #419

Deliverables

• Delete the Object testInstance parameter from TestInstancePostProcessor.postProcessTestInstance(Object, ExtensionContext).
• Update all demo implementations accordingly.
• Update Javadoc accordingly.
• Add an entry in the M5 release notes.

[sbrannen]

FYI: the current work on this issue is being performed in the following branch.

https://github.com/junit-team/junit5/tree/issues/910-remove-test-instance-from-TIPP