If you have to have a conditional dispatch in there to figure out if this is a positive or negative test, maybe you should consider splitting the declarative parts of the testing.

Really, though, I would just go for:
<pre>
it "should allow '/' in the middle of a name" do
    task.set(name: 't/1').save
end

it "should reject '/' at the end of name' do
  rejects 'name', 't/'
end
</pre>

...because, honestly, that is clearer and easier to understand than the code as presented.