Adjust error matching predicate during fuzzing (#13206)

Fuzzing over the weekend found a failure where a wasmtime "rhs" failed
with a trap and a wasmtime "lhs" failed with "gc heap not big enough".
This is expected to be effectively equivalent but the method of testing
whether a Wasmtime error is "non deterministic", which running out of GC
heap space basically is, differed slightly with some other code which
led to this differential failure.

Author: alexcrichton

crates/fuzzing/src/oracles/diff_wasmtime.rs

@@ -65,10 +65,7 @@ impl DiffEngine for WasmtimeEngine {
     }
 
     fn is_non_deterministic_error(&self, err: &Error) -> bool {
-        match err.downcast_ref::<Trap>() {
-            Some(trap) => super::wasmtime_trap_is_non_deterministic(trap),
-            None => false,
-        }
+        super::wasmtime_error_is_non_deterministic(err)
     }
 }