Run Python unittests and doctests.

Add a meta/check.sh script that runs unit tests before the syntax
linters.

Add unittest converters for the existing doctests.

meta/test_srcgen.py

@@ -0,0 +1,8 @@
+from __future__ import absolute_import
+import doctest
+import srcgen
+
+
+def load_tests(loader, tests, ignore):
+    tests.addTests(doctest.DocTestSuite(srcgen))
+    return tests