Improve test coverage.

tests/trailing_whitespace_fixer_test.py

@@ -0,0 +1,26 @@
+from plumbum import local
+
+from pre_commit_hooks.trailing_whitespace_fixer import fix_trailing_whitespace
+
+
+def test_fixes_trailing_whitespace(tmpdir):
+    with local.cwd(tmpdir.strpath):
+        for filename, contents in (
+            ('foo.py', 'foo \nbar \n'),
+            ('bar.py', 'bar\t\nbaz\t\n'),
+        ):
+            with open(filename, 'w') as file_obj:
+                file_obj.write(contents)
+
+        ret = fix_trailing_whitespace(['foo.py', 'bar.py'])
+        assert ret == 1
+
+        for filename, after_contents in (
+            ('foo.py', 'foo\nbar\n'),
+            ('bar.py', 'bar\nbaz\n'),
+        ):
+            assert open(filename).read() == after_contents
+
+
+def test_returns_zero_for_no_changes():
+    assert fix_trailing_whitespace([__file__]) == 0