[js-api] Add test for preserving identity in rethrow

This adds JS API tests that show `catch`-`rethrow` and
`catch_all`-`rethrow` preserve exception objects' identity.

test/js-api/exception/identity.tentative.any.js

@@ -0,0 +1,61 @@
+// META: global=window,dedicatedworker,jsshell
+// META: script=/wasm/jsapi/assertions.js
+// META: script=/wasm/jsapi/wasm-module-builder.js
+
+test(() => {
+  const tag = new WebAssembly.Tag({ parameters: ["i32"] });
+  const exn = new WebAssembly.Exception(tag, [42]);
+  const exn_same_payload = new WebAssembly.Exception(tag, [42]);
+  const exn_diff_payload = new WebAssembly.Exception(tag, [53]);
+
+  const builder = new WasmModuleBuilder();
+  const jsfuncIndex = builder.addImport("module", "jsfunc", kSig_v_v);
+  const tagIndex = builder.addImportedTag("module", "tag", kSig_v_i);
+  const imports = {
+    module: {
+      jsfunc: function() { throw exn; },
+      tag: tag
+    }
+  };
+
+  builder
+    .addFunction("catch_rethrow", kSig_v_v)
+    .addBody([
+      kExprTry, kWasmStmt,
+        kExprCallFunction, jsfuncIndex,
+      kExprCatch, tagIndex,
+        kExprDrop,
+        kExprRethrow, 0x00,
+      kExprEnd
+    ])
+    .exportFunc();
+
+  builder
+    .addFunction("catch_all_rethrow", kSig_v_v)
+    .addBody([
+      kExprTry, kWasmStmt,
+        kExprCallFunction, jsfuncIndex,
+      kExprCatchAll,
+        kExprRethrow, 0x00,
+      kExprEnd
+    ])
+    .exportFunc();
+
+  const buffer = builder.toBuffer();
+  WebAssembly.instantiate(buffer, imports).then(result => {
+    try {
+      result.instance.exports.catch_rethrow();
+    } catch (e) {
+      assert_equals(e, exn);
+      assert_not_equals(e, exn_same_payload);
+      assert_not_equals(e, exn_diff_payload);
+    }
+    try {
+      result.instance.exports.catch_all_rethrow();
+    } catch (e) {
+      assert_equals(e, exn);
+      assert_not_equals(e, exn_same_payload);
+      assert_not_equals(e, exn_diff_payload);
+    }
+  });
+}, "Identity check");