[BEAM-3250] Migrate Flink ValidatesRunner to Gradle

[bsidhom]

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The ValidatesRunner test configuration lives under runners/flink as in the Maven build. If accepted, this supersedes #4418.

[bsidhom]

r: @lukecwik
@kennknowles

[bsidhom]

Rebased to pull in project name refactoring.

[bsidhom]

r: @lukecwik

[lukecwik]

After looking at this approach and the other approach in #4418, this looks much nicer.