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/fkr.expect.out

@@ -0,0 +1,8 @@
+saw expected startup prompt
+ctrl-a handled
+\xE9, first byte, handled
+\xE9, second byte, handled
+\c@ handled
+invalid terminate sequence handled
+valid terminate sequence handled
+exited on seeing valid terminate