RFR: 8299043 : test/jdk/javax/swing/AbstractButton/5049549/bug5049549.java fails with java.lang.NullPointerException

[lawrence.andrews]

Test was failing with java.lang.NullPointerException due to image file extension was specified in upper case ( GIF ) https://github.com/openjdk/jdk/blob/master/test/jdk/javax/swing/AbstractButton/5049549/bug5049549.java#L44-L50  but actually it is in smaller case ( gif )  https://github.com/openjdk/jdk/tree/master/test/jdk/javax/swing/AbstractButton/5049549 . 
The was working as expected in mac but failed in ubuntu. After fixing the image file extension( gif ) and run the test it worked and exception was not reproduced. 

@shurymury 
@honkar-jdk

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Commit messages:
 - 8299043 : test/jdk/javax/swing/AbstractButton/5049549/bug5049549.java fails with java.lang.NullPointerException

Changes: https://git.openjdk.org/jdk/pull/11731/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11731&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299043
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/11731.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11731/head:pull/11731

PR: https://git.openjdk.org/jdk/pull/11731