<Swing Dev> RFR: 8273043: [TEST_BUG] Automate NimbusJTreeSelTextColor.java

[Alexey Ivanov]

Automated NimbusJTreeSelTextColor.java test which is added under [JDK-8271315](https://bugs.openjdk.java.net/browse/JDK-8271315).

It passes with the recent build of JDK 18.  
It fails with JDK 18 build 7 where [JDK-8266510](https://bugs.openjdk.java.net/browse/JDK-8266510) is not fixed yet:


Exception in thread "main" java.lang.RuntimeException: Unexpected color found: ff000000 at (4, 9);
foreground: ffffffff; background: ff39698a - check image.png
        at NimbusJTreeSelTextColor.checkColors(NimbusJTreeSelTextColor.java:106)
        at NimbusJTreeSelTextColor.main(NimbusJTreeSelTextColor.java:70)


The line is wrapped for readability. In this case the text is black instead of white.

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Commit messages:
 - 8273043: [TEST_BUG] Automate NimbusJTreeSelTextColor.java

Changes: https://git.openjdk.java.net/jdk/pull/5269/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5269&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273043
  Stats: 137 lines in 1 file changed: 37 ins; 60 del; 40 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5269.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5269/head:pull/5269

PR: https://git.openjdk.java.net/jdk/pull/5269