java_export to inherit testonly on its sub-targets

[tsawada]

I was trying to use maven_artifacts via java_export as suggested in #648, and noticed that -pom targets fail to build when they include testonly targets as deps.

I am not sure if this is a good way to resolve the issue (whether making pom target as testonly makes sense) though.

[tsawada]

cc @dmivankov

[dmivankov]

Makes sense to me: testonlywas already being passed (if it is set) from java_export via kwargs to java_library, this PR propagates it to other targets under in maven_export/java_export

[shs96c]

This makes sense to me. Thank you for the PR!