Add test for ImportObject's allow_missing_functions

lib/runtime/tests/allow_missing_functions.rs

@@ -0,0 +1,24 @@
+#[test]
+fn allow_missing() {
+    use wabt::wat2wasm;
+    use wasmer_runtime::{imports, instantiate};
+
+    static WAT: &'static str = r#"
+        (module
+        (type (;0;) (func))
+        (type (;1;) (func (result i32)))
+        (import "env" "ret_err" (func $ret_err (type 0)))
+        (func $get_num (type 1)
+            i32.const 42
+        )
+        (export "get_num" (func $get_num))
+        )
+    "#;
+
+    let wasm = wat2wasm(WAT).unwrap();
+
+    let mut import_object = imports! {};
+    import_object.allow_missing_functions = true;
+
+    assert!(instantiate(&wasm, &import_object).is_ok());
+}