Add a spec test fuzzer for Config (#1509)

* Add a spec test fuzzer for Config

This commit adds a new fuzzer which is intended to run on oss-fuzz. This
fuzzer creates and arbitrary `Config` which *should* pass spec tests and
then asserts that it does so. The goal here is to weed out any
accidental bugs in global configuration which could cause
non-spec-compliant behavior.

* Move implementation to `fuzzing` crate

fuzz/Cargo.toml

@@ -50,5 +50,11 @@ test = false
 doc = false
 required-features = ['binaryen']
 
+[[bin]]
+name = "spectests"
+path = "fuzz_targets/spectests.rs"
+test = false
+doc = false
+
 [features]
 binaryen = ['wasmtime-fuzzing/binaryen']

fuzz/fuzz_targets/spectests.rs

@@ -0,0 +1,9 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use wasmtime_fuzzing::generators::{Config, SpecTest};
+
+fuzz_target!(|pair: (Config, SpecTest)| {
+    let (config, test) = pair;
+    wasmtime_fuzzing::oracles::spectest(config, test);
+});