tr: Should not allow special classes in set2

[Ben Wiederhake]

$ echo -en '1234' | tr -t "1234" "[:blank:]"
tr: when translating, the only character classes that may appear in
string2 are 'upper' and 'lower'
[$? = 1]
$ echo -en '1234' | cargo run tr -t "1234" "[:blank:]"
 	34

(Original issue: #6133)