Drupal 7 does not support HTML5 fields, and thus simpletest for D7 doesn't either. However, you can still use HTML5 with the help of contrib modules. We've discovered that if you do use contrib to use HTML5 fields, our tests fail due to the DrupalTestCase class not supporting HTML5 field types.
Here's a patch that's really more of a backport as this support is already present in D8. Apologies if this is a duplicate, I couldn't find an appropriate issue.