RFR: 8308608: [testbug] Use Util::waitForIdle instead of Toolkit::firePulse in system tests

[Karthik P K]

Made changes to use Util::waitForIdle instead of Toolkit::firePulse in system tests. The test will wait for default value of 10 pulses to complete in the scene before executing the subsequent statements.

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Commit messages:
 - Use Util::waitForIdle instead of Toolkit::firePulse

Changes: https://git.openjdk.org/jfx/pull/1230/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1230&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308608
  Stats: 7 lines in 2 files changed: 2 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1230.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1230/head:pull/1230

PR: https://git.openjdk.org/jfx/pull/1230