Declare versions of test-only dependencies inline instead of in our parent pom.

[copybara-service]

Declare versions of test-only dependencies inline instead of in our parent pom.

Fixes #6654, I hope? If guava and its parent pom don't refer to mockito-core, then guava should no longer affect which version of mockito-core is selected by Gradle. (Really, it "shouldn't" even now, but there's a mismatch between Maven's model and Gradle's that causes it to do so.)

I had initially attempted (in cl/552479838) to declare versions of all our dependencies inline, but that didn't work: We really need dependencyManagement for Gradle purposes, at least until we specify versions for Gradle explicitly.

It would be nice if we could still declare our dependency versions only once, now by using properties. (In fact, my attempt to use properties made me notice that our version of the Error Prone plugin is older than our version of the Error Prone annotations.) However, if we were to make that change, then we'd lose the ability to update dependencies with versions-maven-plugin (update-properties + use-latest-releases), I assume because the properties are declared in one pom.xml and used in another. (It's possible that Dependabot is better about this, but we've had trouble getting it to work with our unusual 2-flavor, Google-repo-source-of-truth setup.) I notice that we have this problem even today with truth.version....

Tested:

$ mvn dependency:tree -Dverbose -X -U &> /tmp/pre

# made changes

$ mvn dependency:tree -Dverbose -X -U &> /tmp/post

$ strip() { cat "$@" | grep -v -e 'Dependency collection stats' -e 'Downloading from' -e 'Progress' -e 'Using connector' -e 'Using transporter' -e 'Using mirror' -e maven-dependency-plugin -e SUCCESS -e 'Total time' -e 'Finished at' | sed -e 's/ (.*managed from.*)//'; }; vimdiff <(strip /tmp/pre) <(strip /tmp/post)

RELNOTES=Changed our Maven project to avoid affecting which version of Mockito our Gradle users see.