Add actual tests around statistics module

Also refactor our statistics module to be a bit smarter and less
namedtuple happy. The Statistic class had no reason to be a tuple,
I have no clue why I wrote it that way last night.

tests/unit/test_statistics.py

@@ -1,5 +1,72 @@
+"""Tests for the statistics module in Flake8."""
+import pytest
+
 from flake8 import statistics as stats
+from flake8 import style_guide
+
+DEFAULT_ERROR_CODE = 'E100'
+DEFAULT_FILENAME = 'file.py'
+DEFAULT_TEXT = 'Default text'
 
 
-def test_nothing():
-    pass
+def make_error(**kwargs):
+    """Create errors with a bunch of default values."""
+    return style_guide.Error(
+        code=kwargs.pop('code', DEFAULT_ERROR_CODE),
+        filename=kwargs.pop('filename', DEFAULT_FILENAME),
+        line_number=kwargs.pop('line_number', 1),
+        column_number=kwargs.pop('column_number', 1),
+        text=kwargs.pop('text', DEFAULT_TEXT),
+        physical_line=None,
+    )
+
+
+def test_key_creation():
+    """Verify how we create Keys from Errors."""
+    key = stats.Key.create_from(make_error())
+    assert key == (DEFAULT_FILENAME, DEFAULT_ERROR_CODE)
+    assert key.filename == DEFAULT_FILENAME
+    assert key.code == DEFAULT_ERROR_CODE
+
+
+@pytest.mark.parametrize('code, filename, args, expected_result', [
+    # Error prefix matches
+    ('E123', 'file000.py', ('E', None), True),
+    ('E123', 'file000.py', ('E1', None), True),
+    ('E123', 'file000.py', ('E12', None), True),
+    ('E123', 'file000.py', ('E123', None), True),
+    # Error prefix and filename match
+    ('E123', 'file000.py', ('E', 'file000.py'), True),
+    ('E123', 'file000.py', ('E1', 'file000.py'), True),
+    ('E123', 'file000.py', ('E12', 'file000.py'), True),
+    ('E123', 'file000.py', ('E123', 'file000.py'), True),
+    # Error prefix does not match
+    ('E123', 'file000.py', ('W', None), False),
+    # Error prefix matches but filename does not
+    ('E123', 'file000.py', ('E', 'file001.py'), False),
+    # Error prefix does not match but filename does
+    ('E123', 'file000.py', ('W', 'file000.py'), False),
+    # Neither error prefix match nor filename
+    ('E123', 'file000.py', ('W', 'file001.py'), False),
+])
+def test_key_matching(code, filename, args, expected_result):
+    """Verify Key#matches behaves as we expect with fthe above input."""
+    key = stats.Key.create_from(make_error(code=code, filename=filename))
+    assert key.matches(*args) is expected_result
+
+
+def test_statistic_creation():
+    """Verify how we create Statistic objects from Errors."""
+    stat = stats.Statistic.create_from(make_error())
+    assert stat.error_code == DEFAULT_ERROR_CODE
+    assert stat.message == DEFAULT_TEXT
+    assert stat.filename == DEFAULT_FILENAME
+    assert stat.count == 0
+
+
+def test_statistic_increment():
+    """Verify we update the count."""
+    stat = stats.Statistic.create_from(make_error())
+    assert stat.count == 0
+    stat.increment()
+    assert stat.count == 1