Added py.test tests to exercise the samples

tests/samples.py

@@ -1,3 +1,7 @@
+"""
+Samples of the various charts. Run this script to generate the reference
+samples.
+"""
 import sys, os
 
 from svg.charts.plot import Plot

tests/test_samples.py

@@ -0,0 +1,9 @@
+import samples
+
+def pytest_generate_tests(metafunc):
+	if "sample" in metafunc.funcargnames:
+		for name, chart in samples.generate_samples():
+			metafunc.addcall(funcargs=dict(sample=chart))
+
+def test_sample(sample):
+	res = sample.burn()