RFR: 8296592: Skip failing test StraightLineTest on Linux

[Kevin Rushforth]

Skip `StraightLineTest` on Linux until [JDK-8296590](https://bugs.openjdk.org/browse/JDK-8296590) is fixed.

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Commit messages:
 - 8296592: Skip failing test StraightLineTest on Linux

Changes: https://git.openjdk.org/jfx/pull/943/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296592
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/943.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/943/head:pull/943

PR: https://git.openjdk.org/jfx/pull/943