fix abs jet test comment

The comment suggests a non-existing (incorrect) test. Update the comment
and extend the test.

Change-Id: I362f0b5a2e81d286b672476bdb4394ed20263eb1

internal/ceres/jet_test.cc

diff --git a/internal/ceres/jet_test.cc b/internal/ceres/jet_test.cc
index c6d2036..4718a20 100644
--- a/internal/ceres/jet_test.cc
+++ b/internal/ceres/jet_test.cc
@@ -479,7 +479,9 @@
     ExpectJetsClose(a, b);
   }
 
-  {  // Check that abs(-x * x) == sqrt(x * x).
+  {  // Check that abs(-x * x) == x * x.
+    ExpectJetsClose(abs(-x * x), x * x);
+    // Check that abs(-x) == sqrt(x * x).
     ExpectJetsClose(abs(-x), sqrt(x * x));
   }
   {