test: improve WASI start() coverage

This commit adds additional test cases to
test-wasi-start-validation.js, which gets the JS test coverage
of start() to 100%.

PR-URL: #30972
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/wasi/test-wasi-start-validation.js

@@ -1,7 +1,7 @@
 // Flags: --experimental-wasi-unstable-preview0
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const { WASI } = require('wasi');
 
@@ -30,3 +30,68 @@ const fixtures = require('../common/fixtures');
     );
   })();
 }
+
+(async () => {
+  const wasi = new WASI();
+  const bufferSource = fixtures.readSync('simple.wasm');
+  const wasm = await WebAssembly.compile(bufferSource);
+  const instance = await WebAssembly.instantiate(wasm);
+  const values = [undefined, null, 'foo', 42, true, false, () => {}];
+  let cnt = 0;
+
+  // Mock instance.exports to trigger start() validation.
+  Object.defineProperty(instance, 'exports', {
+    get() { return values[cnt++]; }
+  });
+
+  values.forEach((val) => {
+    assert.throws(
+      () => { wasi.start(instance); },
+      { code: 'ERR_INVALID_ARG_TYPE', message: /\binstance\.exports\b/ }
+    );
+  });
+})();
+
+(async () => {
+  const wasi = new WASI();
+  const bufferSource = fixtures.readSync('simple.wasm');
+  const wasm = await WebAssembly.compile(bufferSource);
+  const instance = await WebAssembly.instantiate(wasm);
+
+  // Mock instance.exports.memory to bypass start() validation.
+  Object.defineProperty(instance, 'exports', {
+    get() {
+      return {
+        memory: new WebAssembly.Memory({ initial: 1 })
+      };
+    }
+  });
+
+  wasi.start(instance);
+  assert.throws(
+    () => { wasi.start(instance); },
+    {
+      code: 'ERR_WASI_ALREADY_STARTED',
+      message: /^WASI instance has already started$/
+    }
+  );
+})();
+
+(async () => {
+  const wasi = new WASI();
+  const bufferSource = fixtures.readSync('simple.wasm');
+  const wasm = await WebAssembly.compile(bufferSource);
+  const instance = await WebAssembly.instantiate(wasm);
+
+  // Mock instance.exports to bypass start() validation.
+  Object.defineProperty(instance, 'exports', {
+    get() {
+      return {
+        memory: new WebAssembly.Memory({ initial: 1 }),
+        __wasi_unstable_reactor_start: common.mustCall()
+      };
+    }
+  });
+
+  wasi.start(instance);
+})();