Update test-k2 convention to use test suites

[adam-enko]

Summary

• use jvm-test-suites to create create separate targets for descriptor and symbols tests
• update Configurations to use newer Gradle guidelines for declarable/resolvable configurations

Usage

• Run ./gradlew descriptorTest to run only tests tagged with a 'descriptor' JUnit tag
• Run ./gradlew symbolsTest to run only tests tagged with a 'symbol' JUnit tag
• ./gradlew test will run all tests without descriptor or symbol tags (to prevent re-running the same tests twice), and will trigger the descriptorTest and symbolsTest tasks.

Benefits

• Using JVM Test Suites unifies common test configuration for each of the test/descriptorsTest/symbolsTest tasks, so custom tasks aren't required
• Avoiding custom tasks will help with Configuration Cache compliance.
• Tagged tests are run in separate tasks, so the same tests aren't run multiple times with different tasks.
• Remove deprecated behaviour - see Build Scan warning "Relying on the convention for Test.classpath in custom Test tasks has been deprecated."