[test] Fix output of assertSame

For errors, it just printed "Failure: expected <Error()> found
<Error()>" and completely omitted the specific error type and the
message.

The new output is:
Failure:
expected:
Error(Error: my explicit error)
found:
Error(ReferenceError: ffi is not defined)

R=mstarzinger@chromium.org

Change-Id: Ie17a97e4413c4585b9560fd1c408018ee8c06701
Reviewed-on: https://chromium-review.googlesource.com/1092746Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53625}

test/mjsunit/mjsunit.js

@@ -278,7 +278,7 @@ var prettyPrinted;
           case "Object":
             break;
           default:
-            return objectClass + "()";
+            return objectClass + "(" + String(value) + ")";
         }
         // [[Class]] is "Object".
         var name = value.constructor.name;