Add agent integration-test module

[heruan]

This adds an integration-test module to verify that the instrumentation agent loads without exceptions when attached to a running application.

• [x] Add test sub-modules
• [x] Configure Jetty to run with the agent attached
• [x] Test that a simple view loads to ensure no exceptions are thrown (e.g. classpath issues, see #148)
• [x] Add more robust tests

[heruan]

What about adding a dependencyMangement section in observability-kit-test modules and importing testbench and junit boms there?

Good idea! I added the BOMs to the root module since they're also used in the agent and starter modules.

Also, we may rename the module to the pluralized form observability-kit-tests

I used the infinite form that we also use in the Collaboration Kit project.