use __accent to better differentiate between a real group.

t/section.vim

@@ -32,7 +32,7 @@ describe 'section'
           \ 'accent': 'red',
           \ })
     let s = airline#section#create(['hi'])
-    Expect s == '%#airline_accent_red#hello'
+    Expect s == '%#__accent_red#hello'
   end
 
   it 'should parse out a section from the distro'