Do fewer classpath calculations in `junit_run`.
Do fewer classpath calculations in `junit_run`.

- Even when coverage is not enabled, we compute a global classpath for junit to pass to `def instrument`. Make this lazy.
- In the case of a single target owning N tests, the batch would contain N tests for the same target. We would then re-calculate the same classpath repeatedly. Dedupe targets.

Testing Done:
https://github.com/pantsbuild/pants/pull/1929

Reviewed at https://rbcommons.com/s/twitter/r/2576/

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