RFR: 8296590: StraightLineTest fails always on Linux and sometimes on other platforms

[Jay Bhaskar]

Issue: The test fails, in case snapshot is taken before web view finish web content load
Solution: Introduce a sleep , before taking snapshot of web view , to test straight line

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Commit messages:
 - Fix straight line test, introduce a sleep before taking snapshot

Changes: https://git.openjdk.org/jfx/pull/1115/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296590
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1115.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1115/head:pull/1115

PR: https://git.openjdk.org/jfx/pull/1115