Register new tasks after plugin is configured.

[thetoothpick]

Avoids NPE when trying to check whether a project is affected while creating tasks.

fixes #287

Context

After #270, the plugin registers new tasks during initial setup, then configures the plugin and initializes the AffectedModuleDetector after projects are evaluated. This fixed #205, but registering the test plugins requires the AffectedModuleDetector, which is null until AffectedModuleDetector.configure(project) is called.

registerCustomTasks() already wrapped its setup in rootProject.afterEvaluate, but registerTestTasks() did not. The execution order is important here, so the register functions were moved after AffectedModuleDetector.configure(), after projects are evaluated.

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