RFR: 8266811: Openjfx controls test build broken (Eclipse)

[Jeanette Winzenburg]

was introduced with fix [JDK-8189354](https://bugs.openjdk.java.net/browse/JDK-8189354) which uses MockListObserver in tests. Fix is analogous to previous eclipse build problems (f.i. [JDK-8265513](https://bugs.openjdk.java.net/browse/JDK-8265513)): add-exports to allow access to base' test package.

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Commit messages:
 - 8266811: Openjfx controls test build broken (Eclipse)

Changes: https://git.openjdk.java.net/jfx/pull/493/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=493&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266811
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/493.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/493/head:pull/493

PR: https://git.openjdk.java.net/jfx/pull/493