Add new Pangram tests

Fixes exercism#79

exercises/pangram/PangramTests.elm

@@ -26,6 +26,30 @@ tests =
           False
           (isPangram "a quick movement of the enemy will jeopardize five gunboats")
         )
+    , test
+        "another missing character 'x'"
+        (assertEqual
+          False
+          (isPangram "the quick brown fish jumps over the lazy dog")
+        )
+    , test
+        "pangram with underscores"
+        (assertEqual
+          True
+          (isPangram "the_quick_brown_fox_jumps_over_the_lazy_dog")
+        )
+    , test
+        "pangram with numbers"
+        (assertEqual
+          True
+          (isPangram "the 1 quick brown fox jumps over the 2 lazy dogs")
+        )
+    , test
+        "missing letters replaced by numbers"
+        (assertEqual
+          False
+          (isPangram "7h3 qu1ck brown fox jumps ov3r 7h3 lazy dog")
+        )
     , test
         "pangram with mixed case and punctuation"
         (assertEqual