Fixes signage issue in discrimination index on Quiz Stats

Indicates both positive and negative with respect to the threshold and
now indicates + and - with respect to 0 (like numbers should)
independently.

Closes CNVS-18382

Test Plan:
  - Create student responses for a quiz to create a discrimination index
    of 0.25>x>0.0 and 0.0>x for two questions
  - See that discrimination index shows red and positive, and red and
    negative respectively, rather than being red and negative and red
    and negative as was reported.

Change-Id: Iebd1eae671d2c7738069728714e934aea10aa717
Reviewed-on: https://gerrit.instructure.com/49384
Tested-by: Jenkins
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>

Author: ryanmt

client_apps/canvas_quizzes/apps/statistics/js/views/charts/discrimination_index.jsx

@@ -36,11 +36,12 @@ define(function(require) {
 
     render: function() {
       var di = this.props.discriminationIndex;
-      var sign = di > K.DISCRIMINATION_INDEX_THRESHOLD ? '+' : '-';
+      var passing = di > K.DISCRIMINATION_INDEX_THRESHOLD ? '+' : '-';
+      var sign = di > 0 ? '+' : '-';
       var className = {
         'index': true,
-        'positive': sign === '+',
-        'negative': sign !== '+'
+        'positive': passing === '+',
+        'negative': passing !== '+'
       };
 
       // TODO: it would be nice if we move this to the model layer so we don't

client_apps/canvas_quizzes/apps/statistics/test/unit/views/charts/discrimination_index_test.js

@@ -35,11 +35,19 @@ define(function(require) {
       expect(find('.index').className).toMatch('negative');
     });
 
-    it('shows a "-" sign when positive', function() {
+    it('shows a "+" sign when below the threshold and above 0', function() {
       setProps({
         discriminationIndex: K.DISCRIMINATION_INDEX_THRESHOLD - 0.1
       });
 
+      expect(find('.sign').innerText).toEqual('+');
+    });
+
+    it('shows a "-" sign when below 0', function(){
+      setProps({
+        discriminationIndex: -0.1
+      });
+
       expect(find('.sign').innerText).toEqual('-');
     });