tr: Should not accept unaligned [:upper:]

[BenWiederhake]

$ echo -en '123456789' | tr -t "123456789" "[:upper:]"
tr: misaligned [:upper:] and/or [:lower:] construct
[$? = 1]
$ echo -en '123456789' | cargo run tr -t "123456789" "[:upper:]"
00000000  41 42 43 44 45 46 47 48  49                       |ABCDEFGHI|
00000009
$

Note that this really checks for alignment, not just presence:

$ echo -en '123abcABC' | tr -t "1[:lower:]" "[:upper:]_"
tr: misaligned [:upper:] and/or [:lower:] construct
[$? = 1]
$ echo -en '123abcABC' | cargo run tr -t "1[:lower:]" "[:upper:]_"
A23BCDABC

(Original issue: #6133)