[foreign-preview] RFR: 8281147: Other tests that use the foreign API are missing --enable-preview

[Julia Boes]

This change adds the --enable-preview flag to tests in other areas (jdk.incubator.vector, java.util.stream) that use the (to be previewed) foreign API, as well as cleaning up the use of a no longer needed proxy class.

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Commit messages:
 - add --enable-Preview to other tests that use the foreign API

Changes: https://git.openjdk.java.net/panama-foreign/pull/636/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=636&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281147
  Stats: 41 lines in 33 files changed: 34 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/636.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/636/head:pull/636

PR: https://git.openjdk.java.net/panama-foreign/pull/636