Can you try to delete in the CLI by escaping the space with a backslash \?
(MM01) [Subtest1] (config) #ap-group "Test\ group
(MM01) ^[Subtest1] (AP group ""Test\ group") #
(MM01) ^[Subtest1] (AP group ""Test\ group") #exit
(MM01) ^[Subtest1] (config) #no ap-group "Test\ group
(MM01) ^[Subtest1] (config) #
If that doesn't work, place the whole between ' :
(MM01) [Subtest1] (config) #show configuration effective | include Test
ap-group '"Test\ group'
(MM01) [Subtest1] (config) #no ap-group '"Test\ group'
(MM01) ^[Subtest1] (config) #