Switch test to using our printf

This test wants to generate a U+FDD2 to see it is not mishandled.

To do so, we tried to use sh, which on my system is bash and can do
`$'\ufdd2'`.

Unfortunately on other systems it might be dash, which won't do that.

Since I don't know of a good no-dependency portable way to generate
this (I dimly remember python3 being a shim on some systems, so I do
not want to invoke it here), we'll just use our own printf.

Which is a worse test, we control both parts, but it'll do.

Fixes #10134

tests/checks/basic.fish

@@ -592,8 +592,8 @@ $fish -c 'echo \ufdd2"fart"'
 # CHECKERR: echo \ufdd2"fart"
 # CHECKERR: ^~~~~~~~~~~^
 
-echo (sh -c 'printf \$\'\ufdd2foo\'') | string escape
-# CHECK: \$\\ufdd2foo
+echo (printf '\ufdd2foo') | string escape
+# CHECK: \Xef\Xb7\X92foo
 
 printf '%s\n' "#!/bin/sh" 'echo $0' > $tmpdir/argv0.sh
 chmod +x $tmpdir/argv0.sh