Proposed testing-common improvements and refactorings

[mateuszrzeszutek]

• Reuse AssertJ-based assertions from the SDK testing module. This probably requires adding a bit of functionality to the SDK assertions first (there probably are some asserts that we have here and they're missing from the SDK testing module). Ideally in this repo we should only have thin Groovy wrappers over the SDK testing assertions.
• Migrate as many test utilities from Groovy to Java (e.g. TraceUtils)
• Refactor InMemoryExporter: it's not really an exporter, it only provides the waitForTraces() method. We should add a shortcut method to InstrumentationSpecification/InstrumentationExtension classes so that direct calls to waitForTraces() are eliminated. This is already done
• Package structure:
  • io.opentelemetry.testing should be the general base package for all testing classes and utilities
  • All spock-related Groovy code should be in the package io.opentelemetry.testing.spock (base classes, assertion wrappers)
  • Javaagent bridging should be in the package io.opentelemetry.testing.javaagent

[trask]

Closing, this is being done as part of #7195.