Add gradle/maven sync parts + restructure tests

[loosebazooka]

Towards #2901

Adds in functions to get the correct sync command. Refactor out some code to share the generation of build args in maven/gradle.

This PR also heavily restructures the jib maven/gradle tests to isolate testing to the methods under test while offering some rudimentary "mockito style" verify to ensure parameters are passed though.

[codecov]

Codecov Report

Merging #3474 into master will decrease coverage by <.01%.
The diff coverage is 91.3%.

Impacted Files	Coverage Δ
pkg/skaffold/build/jib/jib.go	72.38% <0%> (-1.1%)	⬇️
pkg/skaffold/build/jib/gradle.go	100% <100%> (ø)	⬆️
pkg/skaffold/build/jib/maven.go	100% <100%> (ø)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	65.85% <0%> (-2.44%)	⬇️

[briandealwis]

LGTM. Two things I thought of when looking through:

1. Having xxxArgs() and xxxArgsFunc() seems a bit error-prone.
2. Would it make sense to rename Generate{Maven|Gradle}Args to Generate{Maven|Gradle}BuildArgs? It seemed a bit odd to see them call mavenBuildArgsFunc

[loosebazooka]

Yeah, I think my naming is pretty poor on those methods, I'll go look at renaming it.

[loosebazooka]

@briandealwis Actually touching those names makes us also touch the gcb package. I'll do it in a followup.