tests: Fix escaping in bundle tests on Django 3.0

tests: Fix escaping in bundle tests on Django 3.0

Django 3.0 switches to using Python 3's built-in HTML escaper, which
prefers to escape entities using hex rather than decimal.

Some of our tests check rendered HTML output against pre-escaped
strings, and fail because ''' is now '''.

Fix this by using the escape function so we get consistent escaping no
matter which Django version.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>