DebugTools: use Debug instead of Error for testing comparator output.

Otherwise unrelated error messages such as file read failures would get
included, making the tests fail. The Debug output on the other hand
isn't used for any internal reporting.

src/Magnum/DebugTools/Test/CompareImageTest.cpp

@@ -591,8 +591,8 @@ void CompareImageTest::compareDifferentSize() {
         TestSuite::ComparisonStatusFlags flags = compare(a, b);
         /* No diagnostic as we don't have any expected filename */
         CORRADE_COMPARE(flags, TestSuite::ComparisonStatusFlag::Failed);
-        Error e(&out);
-        compare.printMessage(flags, e, "a", "b");
+        Debug d{&out};
+        compare.printMessage(flags, d, "a", "b");
     }
 
     CORRADE_COMPARE(out.str(), "Images a and b have different size, actual Vector(3, 4) but Vector(3, 5) expected.\n");
@@ -610,8 +610,8 @@ void CompareImageTest::compareDifferentFormat() {
         TestSuite::ComparisonStatusFlags flags = compare(a, b);
         /* No diagnostic as we don't have any expected filename */
         CORRADE_COMPARE(flags, TestSuite::ComparisonStatusFlag::Failed);
-        Error e(&out);
-        compare.printMessage(flags, e, "a", "b");
+        Debug d{&out};
+        compare.printMessage(flags, d, "a", "b");
     }
 
     CORRADE_COMPARE(out.str(), "Images a and b have different format, actual PixelFormat::RGBA32F but PixelFormat::RGB32F expected.\n");