don't allow f-k-r to run if stdin/stdout not a tty

Another developer noticed that redirecting stdin of `fish_key_reader`
results in weird behavior.  Which is not at all surprising. So add checks
to ensure stdin and stdout are attached to a tty.

Add some rudimentary unit tests for this program.

tests/interactive.expect.rc

@@ -4,6 +4,7 @@ log_user 0
 log_file -noappend interactive.tmp.log
 
 set fish ../test/root/bin/fish
+set fish_key_reader ../test/root/bin/fish_key_reader
 
 set timeout 5