Improve stability for fuzz targets. (#3804)

This commit improves the stability of the fuzz targets by ensuring the
generated configs and modules are congruent, especially when the pooling
allocator is being used.

For the `differential` target, this means both configurations must use the same
allocation strategy for now as one side generates the module that might not be
compatible with another arbitrary config now that we fuzz the pooling
allocator.

These changes also ensure that constraints put on the config are more
consistently applied, especially when using a fuel-based timeout.

fuzz/fuzz_targets/differential_wasmi.rs

@@ -13,9 +13,8 @@ fuzz_target!(|data: &[u8]| {
 fn run(data: &[u8]) -> Result<()> {
     let mut u = Unstructured::new(data);
     let mut config: generators::Config = u.arbitrary()?;
-    config.module_config.set_differential_config();
-    let mut module = config.module_config.generate(&mut u)?;
-    module.ensure_termination(1000);
+    config.set_differential_config();
+    let module = config.generate(&mut u, Some(1000))?;
     oracles::differential_wasmi_execution(&module.to_bytes(), &config);
     Ok(())
 }