Add test coverage for parsers in tools/lib.

Now, `tools/test-all` calls a new program called `tools/tests-tools`
that runs unit tests in `test_css_parser.py` and 'test_template_parser.py`.

This puts 100% line coverage on tools/lib/css_parser.py.
This puts about 50% line coverage on tools/lib/template_parser.py.

tools/tests/test_template_parser.py

@@ -0,0 +1,34 @@
+from __future__ import absolute_import
+from __future__ import print_function
+
+import sys
+import unittest
+
+try:
+    from tools.lib.template_parser import (
+        is_django_block_tag,
+        validate,
+    )
+except ImportError:
+    print('ERROR!!! You need to run this via tools/test-tools.')
+    sys.exit(1)
+
+class ParserTest(unittest.TestCase):
+    def test_is_django_block_tag(self):
+        # type: () -> None
+        self.assertTrue(is_django_block_tag('block'))
+        self.assertFalse(is_django_block_tag('not a django tag'))
+
+    def test_validate_vanilla_html(self):
+        # type: () -> None
+        '''
+        Verify that validate() does not raise errors for
+        well-formed HTML.
+        '''
+        my_html = '''
+            <table>
+                <tr>
+                <td>foo</td>
+                </tr>
+            </table>'''
+        validate(text=my_html)