Bug 1603172 - Increase fuzz on split-batch.yaml test.

This should get it passing github CI on macOS release builds.

Differential Revision: https://phabricator.services.mozilla.com/D56779

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/334e965549ada90f222ba0138bd8aeda63c34750

wrench/reftests/text/reftest.list

@@ -16,7 +16,7 @@ skip_on(android,device) == shadow.yaml shadow-ref.yaml  # Fails on Pixel2
 == decorations.yaml decorations-ref.yaml
 skip_on(android,device) fuzzy(1,3001) == decorations-suite.yaml decorations-suite.png  # Fails on Pixel2
 == 1658.yaml 1658-ref.yaml
-fuzzy(1,5) == split-batch.yaml split-batch-ref.yaml
+fuzzy(1,6) == split-batch.yaml split-batch-ref.yaml
 # Next 3 tests affected by bug 1548099 on Android
 skip_on(android) == shadow-red.yaml shadow-red-ref.yaml
 skip_on(android) fuzzy(1,735) == shadow-grey.yaml shadow-grey-ref.yaml