RFR: 8367306: HeaderButtonOverlayTest fails to compile with TEST_SDK_PATH

[Ambarish Rapte]

When using `-PTEST_SDK_PATH=<path>` and `-PTEST_ONLY=true`, the test `HeaderButtonOverlayTest` fails to compile with error: _package com.sun.glass.events is not visible_

The test compiles and executes fine with normal command: `gradle :graphics:test`.
In this case the JavaFX is built locally and the `com.sun.glass.events.MouseEvent` class file becomes reachable through classpath. A bug is filed to investigate this separately : [JDK-8367327](https://bugs.openjdk.org/browse/JDK-8367327)

Fix is quick, to update the `modules/javafx.graphics/src/test/addExports`.

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Commit messages:
 - 8367306: HeaderButtonOverlayTest fails to compile with TEST_SDK_PATH

Changes: https://git.openjdk.org/jfx/pull/1891/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1891&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367306
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1891.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1891/head:pull/1891

PR: https://git.openjdk.org/jfx/pull/1891