RFR: 8349679: build.gradle: increase system test memory limit to 1GB

[Andy Goryachev]

Increased max heap size for system tests from 521Mb to 1000Mb.

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Commit messages:
 - increase max heap size for system tests

Changes: https://git.openjdk.org/jfx/pull/1701/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1701&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349679
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1701.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1701/head:pull/1701

PR: https://git.openjdk.org/jfx/pull/1701