Skip fixture comparisons and remaining tests if compilation failed

If the test is supposed to compile, but compilation fails instead,
there's no point in diffing fixtures (unless you like a wall of red in
your terminal). It also turns out that execution never reaches the
diffing section, since writing the non-existent debug Wasm binary fails.

tests/compiler.js

@@ -321,6 +321,10 @@ async function runTest(basename) {
       }
       compareStderr.end(SUCCESS);
       return prepareResult(SUCCESS);
+    } else if (error) {
+      // Don't bother comparing fixtures or doing anything else if the
+      // compilation failed.
+      return prepareResult(FAILURE, "compile failed");
     }
 
     const afterCompileResult = afterCompile("debug");