Introduce methods to change SerializerRegistry used by test workers.

Context

I am working on a plugin that supports testing Scala and ScalaJS code using test frameworks that implement sbt.testing.Framework.

I need to convey to the test workers, in addition to the test class name, framework-specific information obtained during test discovery. To do that, I need to use a parameter serializer other than the currently hard-coded TestEventSerializer.

Proposed change satisfies this need; without it, I will have to include in my plugin modified versions of the three Gradle classes - hundreds of lines of code!

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