fixes #184265

This fixes an oddity related to mmap files. A delay had to be introduced in the test because of oddities with the filesystem. I tried a few tricks to get it to work without the delay but this is the only way I could get it to work.

testing: has existing test, removes a flake

I tested it manually locally:

for i in {1..30}; do echo "Running iteration: $i"; ./testing/run_tests.py --type=dart --dart-filter=shader_reload_test.dart --variant host_debug_unopt_arm64; done

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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