The rasterizer may only be accessed safely on the GPU task runner. The test was accessing the same on a non-engine known task runner instead (i.e the tests main task runner).
Crashes previously reproducible on all platforms with the following filters: `--gtest_filter="*ShellTest.SetResourceCacheSize*" --gtest_repeat=-1 --gtest_shuffle --gtest_random_seed=1988` at run ~400.
Fixes https://github.com/flutter/flutter/issues/37629