Fuzzer: Run Asyncify less

[kripken]

https://github.com/WebAssembly/binaryen/issues/4865 is not new, and will likely show up more. As the actual bug is likely very old (there have been no changes to Asyncify for a while) this is low priority. Run Asyncify less in the fuzzer for now (0.6 meant it ran 60% of the time, which is too high anyhow).