change distance tests to use unittest

The exact wording and text in the "Fix the error" section need updating,
but the provided code will now work in CodeLens with the current
PythonTutor version.

Fixes #279.

Author: dandrake

pretext/Functions/ProgramDevelopment.ptx

@@ -31,11 +31,17 @@ def distance(x1, y1, x2, y2):
   <p>We import the test module to enable us to write a unit test for the function.</p>
   <program xml:id="ch06_distance1" interactive="activecode" language="python">
     <code>
-import test
+import unittest
+
 def distance(x1, y1, x2, y2):
     return 0.0
 
-test.testEqual(distance(1, 2, 1, 2), 0)
+class TestDistance(unittest.TestCase):
+    def test_distance(self):
+        self.assertEqual(distance(1, 2, 1, 2), 0)
+
+x = TestDistance('test_distance'))
+x.run()
         </code>
   </program>
   <p>The <c>testEqual</c> function from the test module calls the distance function with sample inputs: (1,2, 1,2).
@@ -85,17 +91,33 @@ def distance(x1, y1, x2, y2):
             we compute and return the result.</p>
   <program xml:id="ch06_distancefinal" interactive="activecode" language="python">
     <code>
-import test
+import unittest
+
 def distance(x1, y1, x2, y2):
     dx = x2 - x1
     dy = y2 - y1
     dsquared = dx**2 + dy**2
     result = dsquared**0.5
     return result
 
-test.testEqual(distance(1,2, 1,2), 0)
-test.testEqual(distance(1,2, 4,6), 5)
-test.testEqual(distance(0,0, 1,1), 1.41)
+class TestDistance(unittest.TestCase):
+    def test_distance_zero(self):
+        self.assertEqual(distance(1,2, 1,2), 0)
+
+    def test_distance_five(self):
+        self.assertEqual(distance(1,2, 4,6), 5)
+
+    def test_distance_square(self):
+        self.assertEqual(distance(0,0, 1,1), 1.41)
+
+x = TestDistance('test_distance_zero')
+x.run()
+
+x = TestDistance('test_distance_five')
+x.run()
+
+x = TestDistance('test_distance_square')
+x.run()
         </code>
   </program>
   <note>
@@ -117,7 +139,7 @@ test.testEqual(distance(0,0, 1,1), 1.41)
           <p>Copy line 11 on to line 12. On line 12, change <c>1.41421</c> to <c>1.41</c>. Run. The test fails.</p>
         </li>
         <li>
-          <p>Type <c>, 2</c> after 1.41. (The 2 represents the precision of the test &#x2013; how many digits to the right of the decimal that must be correct.) Run.</p>
+          <p>Change the line to <c>self.assertAlmostEqual(distance(0,0, 1,1), 1.41, places=2)</c>. (The <c>assertAlmostequal</c> and <c>places=2</c> represent the precision of the test &#x2013; how many digits to the right of the decimal that must be correct.) Run.</p>
         </li>
       </ul>
     </p>