RFR: 8303911: [testbug] Some unit tests call System.exit

[Lukasz Kostyra]

Changes added to this commit resolve cases where `System.exit()` was used to leave tests abruptly. These tests, if System.exit() is hit, could potentially create some unwanted behavior from JTest (ex. silently skipping these tests instead of failing them).

There were some files which I did not modify, mostly because they were a part of a test sub-app, or tests would require major rework to make it work.

Two FXML tests were performance tests already written as separate apps, so I moved them to appropriate directory in tests/performance.

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Commit messages:
 - JFXPanel tests: Remove main() method in tests
 - ImageRaceTest: Replace System.exit() with Assert.fail()
 - FXML: Move Performance tests to tests/performance
 - StubToolkit: Throw exception when exit() is called
 - graphics: Remove old disabled ImageStorageTest

Changes: https://git.openjdk.org/jfx/pull/1101/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1101&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303911
  Stats: 440 lines in 7 files changed: 1 ins; 435 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1101.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1101/head:pull/1101

PR: https://git.openjdk.org/jfx/pull/1101