Fix JSInput tests

Select DOM elements with wildcard syntax

- DOM lookups were being done with non-existent literal selectors, so it
  was returning empty lists. As assertions were to be made while
  iterating over the list of elements, nothing was actually being
  verified.

- Common code has been centralized in the setup function. By declaring
  CSS selectors once, we minimize the odds of inadvertently using the
  wrong selector, as happened here.

- Had these tests actually been iterating over a non-empty list, this
  would have thrown undefined exceptions. jQuery.each calls its handler
  with an index and an item/object as the arguments. However, the object
  is a DOM-object, not a jQuery-object.  These tests break, as they had
  assumed the latter.

common/static/js/capa/spec/jsinput_spec.js

-describe("JSInput", function() {
+describe("JSInput", function () {
+    var sections;
+    var inputFields;
+
     beforeEach(function () {
         loadFixtures('js/capa/fixtures/jsinput.html');
+        sections = $('section[id^="inputtype_"]');
+        inputFields = $('input[id^="input_"]');
+        JSInput.walkDOM();
     });
 
-    it('sets all data-processed attributes to true on first load', function() {
-        var sections = $(document).find('section[id="inputtype_"]');
-        JSInput.walkDOM();
-        sections.each(function(index, section) {
-            expect(section.attr('data-processed')).toEqual('true');
+    it('sets all data-processed attributes to true on first load', function () {
+        sections.each(function (index, item) {
+            expect(item).toHaveData('processed', true);
         });
     });
 
     it('sets the data-processed attribute to true on subsequent load', function() {
-        var section1 = $(document).find('section[id="inputtype_1"]'),
-            section2 = $(document).find('section[id="inputtype_2"]');
+        var section1 = $(this.sections[0]);
+        var section2 = $(this.sections[1]);
         section1.attr('data-processed', false);
         JSInput.walkDOM();
         expect(section1.attr('data-processed')).toEqual('true');
         expect(section2.attr('data-processed')).toEqual('true');
     });
 
-    it('sets the waitfor attribute to its update function', function() {
-        var inputFields = $(document).find('input[id="input_"]');
-        JSInput.walkDOM();
-        inputFields.each(function(index, inputField) {
-            expect(inputField.data('waitfor')).toBeDefined();
+    it('sets the waitfor attribute to its update function', function () {
+        inputFields.each(function (index, item) {
+            expect(item).toHaveAttr('waitfor');
         });
     });
 
     it('tests the correct number of sections', function () {
-        var sections = $(document).find('section[id="inputtype_"]');
-        var inputFields = $(document).find('input[id="input_"]');
         expect(sections.length).toEqual(2);
         expect(sections.length).toEqual(inputFields.length);
     });