Whoa, this is a long diff. Here some little things I noticed:

Can you move this to the top of the imports, since mock is  part of the Python Standard Library?
And a blank line in-between that and the django/djblets imports, of course.

You didn't make these changes, but shouldn't the docstrings for the tests involving anonymous users says ...with anonymous user instead of ...with authenticated user?
Same below.

I noticed in the tests above that they specify that the user is authenticated. Perhaps you could mention that as well?
Same with the test directly below.

I think you should be more specific with the condition for the unit test here.
So, you should include a mention about the can_change_status permission. Something like Testing CloseMenuAction.should_render for user with can_change_status permission but in read-only mode
Same with some tests below.

I think you forgot to add periods at the end of your comments with these changes.
Same with the other comments in your changes.

Same here (mention the permission?).

Same here (mention the permission?).

Same here (mention the permission?).

Do we want to refactor this into a mixin that does common tests for actions that respect read-only mode?

Closing quote on next line.

LIkewise.

We have test users from the test-users fixture. You can do:

self.request.user = User.objects.get(username='doc')


This user is not a superuser (username admin is).

See above comment about user fixture.

Closing quote on next line.

We are trying to move away from Mock (as it is a rather heavy dependency), although I understand that previous tests in this file are using it.
Ideally all of these would be replaced with a fake resolver match class:

class FakeResolverMatch(object):
    def __init__(self, url_name):
        self.url_name = url_name


This will ultimately result in fewer lines of code.

Missing a period.

Closing quote on next line.

See my previous comment about the user fixture.

See my previous comment about the user fixture.
You can use the admin user here.

We have an admin user:

admin = User.objects.get(username='admin')
review_request = self.create_review_request(
    create_repository=True,
    submitter=admin)
self.request.user = admin

This probably does not really matter, but I think in the previous code you put 'perms' before 'request'. Probably should keep this consistent?

Two blank line between imports and first declaration.

Missing docstring.

Missing docstring.

This should indicate in its name that its a mixin of tests, e.g., ReadOnlyActionTestsMixin.
Missing docstring.

When you do:

class Foo:
    @add_fixtures(['..'])
    def test_foo(self): pass


it is actually equivalent to:
class Foo:
    def test_foo(self): pass
    test_foo = add_fixtures(['..'])(test_foo)


so you should actually replace the functions on the instance:

self.test_should_render_with_user_in_read_only = _add_fixtures(self.test_should_render_with_user_in_read_only)
self.test_should_render_with_superuser_in_read_only = _add_fixtures(self.test_should_render_with_superuser_in_read_only)

Instead of doing this every test, how about...

Use ~reviewboard.reviews.actions.BaseReviewRequestAction and it will render the link with the text of only "BaseReviewRequestAction".

we do:

try:
    self.siteconfig.set('site_read_only', True)
    self.siteconfig.save()

    #... rest of test
finally:
    self.siteconfig.set('site_read_only', defaults.get('site_read_only'))
    self.siteconfig.save()


This prevents some unnecessary siteconfig saving when nothing has changed.
Same for the below test.