RFR: 8285635: javax/swing/JRootPane/DefaultButtonTest.java failed with Default Button not pressed for L&F: com.sun.java.swing.plaf.motif.MotifLookAndFeel

[Prasanta Sadhukhan]

Test sometimes fail for Motif L&F only which might require some code change in Motif code but since Motif is already deprecated and obsolete, it seems prudent to skip Motif L&F in the test.
Several iterations of the modified test pass in all platforms.

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Commit messages:
 - 8285635: javax/swing/JRootPane/DefaultButtonTest.java failed with Default Button not pressed for L&F: com.sun.java.swing.plaf.motif.MotifLookAndFeel

Changes: https://git.openjdk.org/jdk/pull/10978/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10978&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8285635
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10978.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10978/head:pull/10978

PR: https://git.openjdk.org/jdk/pull/10978