<Swing Dev> RFR: 8137101: [TEST_BUG] javax/swing/plaf/basic/BasicHTML/4251579/bug4251579.java failure due to timing

[Prasanta Sadhukhan]

This test was failing in CI test run due to timing issue. Adjusted robot delay..Also, made frame to show in middle of screen and dispose of frame at end.
CI test run is green.

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Commit messages:
 - Fix
 - 8137101: [TEST_BUG] javax/swing/plaf/basic/BasicHTML/4251579/bug4251579.java failure due to timing

Changes: https://git.openjdk.java.net/jdk/pull/4918/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4918&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8137101
  Stats: 51 lines in 2 files changed: 18 ins; 5 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4918.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4918/head:pull/4918

PR: https://git.openjdk.java.net/jdk/pull/4918