Improve setExtended by Removing Pollution in PotionTest

[maythefirst]

This PR aims to improve a test org.bukkit.potion.PotionTest.setExtended, which is non-idempotent and fails if run twice in the same JVM. The failure message is as follows

Tests in error: 
  setExtended(org.bukkit.potion.PotionTest): Cannot set already-set type

Tests run: 2, Failures: 0, Errors: 1, Skipped: 0

The reason for this is because it pollutes states (byId and byName in the main code) shared among tests.
Though it seems to be a minor issue, we propose that it might be good to clean this state pollution after the execution of this test so that some other tests do not fail in the future due to the shared state polluted by this test.

[TheKodeToad]

Not maintained anymore. Updated version (maintained by SpigotMC):
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/browse#contributing