RFR: 8256581: Refactor vector conversion tests

[Paul Sandoz]

Refactor the vector conversions tests to improve performance and reduce explicit test methods (using data providers).

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Commit messages:
 - 8256581: Refactor vector conversion tests

Changes: https://git.openjdk.java.net/jdk/pull/1302/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1302&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256581
  Stats: 37231 lines in 6 files changed: 212 ins; 36768 del; 251 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1302.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1302/head:pull/1302

PR: https://git.openjdk.java.net/jdk/pull/1302