RFR: 8234194: [TEST_BUG] Reenable few graphics unit tests

[Ambarish Rapte]

Following graphics unit tests can be re-enabled.

 1. test.com.sun.javafx.scene.layout.region.BackgroundRepeatConverterTest.scenario2
RepeatStructConverter.convert() is an internal method and does not verify the parameters. Passing null value to this method results in NPE.

2. test.javafx.scene.layout.RegionCSSTest.borderImageWidth_auto
test.javafx.scene.layout.RegionCSSTest.borderImageWidth_1_auto
test.javafx.scene.layout.RegionCSSTest.borderImageWidth_1_2Percent_auto
It is not certain if the value of auto for -fx-border-image-width is 1 or 2 by defaut. But the tests seem correct and they pass.

3. test.javafx.scene.Node_cssMethods_Test
[JDK-8094155](https://bugs.openjdk.java.net/browse/JDK-8094155) is fixed and the tests can be re-enabled.

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Commits:
 - 5df00884: Reenable few of the graphics unit tests

Changes: https://git.openjdk.java.net/jfx/pull/39/files
 Webrev: https://webrevs.openjdk.java.net/jfx/39/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234194
  Stats: 12 lines in 3 files changed: 0 ins; 7 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/39.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/39/head:pull/39

PR: https://git.openjdk.java.net/jfx/pull/39