core-construction-java should not depend on testing

core-construction-java currently depends on a class in the testing package because runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/TestStreamTranslation.java registers a translation for org.apache.beam.sdk.testing.TestStream.

This means that when using core-construction-java, one needs to also depend on the testing code. Core should not have to depend on testing, the translation should move to the testing package.

This also affects the direct runner with runners/direct-java/src/main/java/org/apache/beam/runners/direct/TestStreamEvaluatorFactory.java. For the direct runner, we should have a way to register these factories at run time (via Autoservice) so that this can also be moved to the testing package.

Imported from Jira [BEAM-3495](https://issues.apache.org/jira/browse/BEAM-3495). Original Jira may contain additional context.
Reported by: jeremiele_g.