Limit the number of exported function calls we make in the API calls fuzzer

This should fix some fuzzing timeouts like
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20847

crates/fuzzing/src/generators/api.rs

@@ -48,12 +48,20 @@ use ApiCall::*;
 #[derive(Default)]
 struct Scope {
     id_counter: usize,
-    predicted_rss: usize,
+
     /// Map from a module id to the predicted amount of rss it will take to
     /// instantiate.
     modules: BTreeMap<usize, usize>,
+
     /// Map from an instance id to the amount of rss it's expected to be using.
     instances: BTreeMap<usize, usize>,
+
+    /// The rough predicted maximum RSS of executing all of our generated API
+    /// calls thus far.
+    predicted_rss: usize,
+
+    /// The number of calls of an exported function from an instance.
+    num_export_calls: usize,
 }
 
 impl Scope {
@@ -83,6 +91,11 @@ impl Arbitrary for ApiCalls {
         let mut scope = Scope::default();
         let max_rss = 1 << 30; // 1GB
 
+        // Calling an exported function of a `wasm-opt -ttf` module tends to
+        // take about 20ms. Limit their number to 100, or ~2s, so that we don't
+        // get too close to our 3s timeout.
+        let max_export_calls = 100;
+
         for _ in 0..input.arbitrary_len::<ApiCall>()? {
             let mut choices: Vec<fn(_, &mut Scope) -> arbitrary::Result<ApiCall>> = vec![];
 
@@ -122,8 +135,12 @@ impl Arbitrary for ApiCalls {
                     Ok(InstanceDrop { id })
                 });
             }
-            if swarm.call_exported_func && !scope.instances.is_empty() {
+            if swarm.call_exported_func
+                && scope.num_export_calls < max_export_calls
+                && !scope.instances.is_empty()
+            {
                 choices.push(|input, scope| {
+                    scope.num_export_calls += 1;
                     let instances: Vec<_> = scope.instances.keys().collect();
                     let instance = **input.choose(&instances)?;
                     let nth = usize::arbitrary(input)?;