Emit dumpInfo string when processor opt test fails

Add missing implementation for ComposeTwo, and provide more information in
ComposeOne.

BUG=skia:

Change-Id: Id2406bcb51a6419a763ff9e6e572f5c6c70c4c76
Reviewed-on: https://skia-review.googlesource.com/9699
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/tests/ProcessorTest.cpp b/tests/ProcessorTest.cpp
index e2eeeb9..5c2efd1 100644
--- a/tests/ProcessorTest.cpp
+++ b/tests/ProcessorTest.cpp
@@ -412,6 +412,9 @@
                                fp->name(), input, output);
                         passing = false;
                     }
+                    if (!passing) {
+                        ERRORF(reporter, "Processor details: %s", fp->dumpInfo().c_str());
+                    }
                 }
             }
         }