[MLIR][test] Re-disable FileCheck on async.mlir integration test (#190702)
#190563 re-enabled FileCheck on `Integration/GPU/CUDA/async.mlir`, but
the buildbot has shown intermittent wrong-output failures
([example](https://lab.llvm.org/buildbot/#/builders/116/builds/27026)):
the test produces `[42, 42]` instead of the expected `[84, 84]`.
This wrong-output flakiness is distinct from the cleanup-time
`cuModuleUnload` errors that #190563 actually fixes — it's the
underlying issue tracked by #170833. The merged commit message for
#190563 incorrectly says `Fixes #170833`; that issue should be reopened,
since the cleanup-error fix doesn't address the wrong-output behavior.
This PR puts the test back in its previously-disabled state. The runtime
cleanup fix in #190563 is unaffected.