Add tests for 'a' vs. 'an' rule

Closes #31.

tests/test_mau_a_vs_an.py

@@ -0,0 +1,36 @@
+from check import Check
+from proselint.checks import mau_a_vs_an as chk
+
+
+class TestCheck(Check):
+
+    __test__ = True
+
+    @property
+    def this_check(self):
+        return chk
+
+    def test_clean(self):
+        assert self.check(
+            """An apple a day keeps the doctor away."""
+        )
+
+    def test_a_apple(self):
+        assert not self.check(
+            """A apple a day keeps the doctor away."""
+        )
+
+    def test_an_day(self):
+        assert not self.check(
+            """An apple an day keeps the doctor away."""
+        )
+
+    def test_linebreak(self):
+        assert not self.check(
+            """An apple an\nday keeps the doctor away."""
+        )
+
+    def test_mid_word(self):
+        assert self.check(
+            """The Epicurean garden."""
+        )