DGPv2 Android integration tests

Create integration tests for using DGPv2 with Android projects.

Additionally, create a JUnit extension for helping set up and run the tests.

Part of KT-70855 - the HTML currently contains errors, but this will be resolved in later PRs.

DokkaGradlePluginTestExtension is a JUnit extension for automatically configuring and re-running tests. The behaviour can be configured by providing custom GradleTestProjectInitializer (for setting up the test projects), or custom GradlePropertiesProvider (for providing custom Gradle properties).
The extension will re-run the tests multiple times with different versions. TestedVersions.testedVersions() provides all possible, valid combinations of versions used to run the tests.
Add JUnit @Tag annotations, to help organise and categorise the tests.
Add two new test projects in dokka-integration-tests/gradle/projects: it-android and it-android-compose. Each project has an expectedData directory containing the expected generated Dokka content.
~Split GradlePropertiesBuilder into separate class, so it can be re-used in the integration tests~ Moved to separate PR https://github.com/Kotlin/dokka/pull/3906

[!TIP] This PR contains the expected HTML files, which make the PR difficult to view in GitHub. You can filter out the files in the GitHub UI:

Kotlin / dokka