Support each registered JVM test suite

Currently, the plugin seems to be limited to unit tests, i.e. Gradle's built-in `test` source set. However, more [test suites](https://docs.gradle.org/current/userguide/jvm_test_suite_plugin.html) might have been registered, and thus making the source set of the tests to distribute configurable would be great.

Start by reading the linked Gradle JVM Test Suite Plugin documentation, then locate where the plugin currently assumes Gradle’s built-in `test` source set. Determine how a configurable test source set should select each registered JVM suite; done means registered suites can be chosen for distribution rather than only `test`.

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