[wasm] Slightly improve async tests

These tests are also used for streaming. This CL changes the test to
actually output the exception that unexpectedly happened.

R=ahaas@chromium.org

Bug: v8:7921
Change-Id: Ia9c91a7d3d9452f9c0180329a5434f049b56c3c8
Reviewed-on: https://chromium-review.googlesource.com/c/1319755
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57284}

test/mjsunit/wasm/async-compile.js

@@ -7,17 +7,18 @@
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
-function assertCompiles(buffer) {
-  return assertPromiseResult(
-      WebAssembly.compile(buffer),
-      module => assertTrue(module instanceof WebAssembly.Module),
-      ex => assertUnreachable());
+async function assertCompiles(buffer) {
+  var module = await WebAssembly.compile(buffer);
+  assertInstanceof(module, WebAssembly.Module);
 }
 
-function assertCompileError(buffer) {
-  return assertPromiseResult(
-      WebAssembly.compile(buffer), module => assertUnreachable(),
-      ex => assertTrue(ex instanceof WebAssembly.CompileError));
+async function assertCompileError(buffer) {
+  try {
+    await WebAssembly.compile(buffer);
+    assertUnreachable();
+  } catch (e) {
+    if (!(e instanceof WebAssembly.CompileError)) throw e;
+  }
 }
 
 assertPromiseResult(async function basicCompile() {