LLVM conversion: harden a test to check for LLVM funcs rather than any funcs

This test was not updated in the original commit that switched to using LLVM
functions since it wasn't broken by that change. FileCheck was able to match
the `func` part of `llvm.func` to the expected pattern and continue as usual.
Make sure the `llvm.` dialect prefix is included in the expected output.

PiperOrigin-RevId: 274127281