Recently we've been seeing an increasing number of CI test failures due to this test and the rolls frequently fail due to this test.

For example, this recent run on the dashboard failed on this test on all 3 of its attempts:

https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20Production%20Engine%20Drone/921374/overview

And here is an example of a Skia roll that failed due to this test:

#183722

The Dart SDK roll has also failed on this issue:

#183689

And the Fuchsia Linux SDK roller:

#183721

Running the test manually on my Macbook Pro shows that at times the test has about a 50% failure rate when run singly (i.e. singling it out with (cd out/host_debug_unopt_arm64; ./impeller_golden_tests --gtest_filter="*ShimmerTest*/MetalSDF" --working_dir=../goldens/001)) but other times will pass many times in a row.