Relaxed tolerance allowing two failing render tests to pass on GL-Native linux

[SnailBones]

Launch Checklist

On running:

./build/bin/mbgl-render-test-runner -p metrics/linux-clang8-release-style.json

Before:

After:

1392 passed (84.9%)
79 passed but were ignored (4.8%)
168 ignored (10.3%)

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