Remove testing-common dependency on guava

[mateuszrzeszutek]

Is your feature request related to a problem? Please describe.
Right now testing-common depends on guava 20 and it's impossible to update the guava version because several instrumentations use/depend on this version (cassandra, grizzly, ratpack, etc).
With #1643 being merged the javaagent code now uses a separate, shaded guava that will not interfere with the older version used by the instrumented lib (and if it does then it's a bug).
After we remove the test dependency on guava javaagent will use its own version, the instrumented library under test will use its own and we'll be free to upgrade the javaagent one to the newest version.