t5563: prevent "ambiguous redirect"

t5563: prevent "ambiguous redirect"

When I ran this test using `TEST_SHELL_PATH=/bin/bash` in my Ubuntu
setup (where Bash is at version 5.0.17(1)-release), I was greeted with
this error message:

	./test-lib.sh: line 1072: $CHALLENGE: ambiguous redirect

This commit fixes that error by quoting the `CHALLENGE` variable (which
has as value a path containing spaces), and by avoiding to cuddle the
empty string parameter in the `printf` call with the redirect character
(in fact, the `printf ''>$CHALLENGE` is removed because the next line
overwrites the file anyway because it _also_ uses a single `>` to
redirect the output).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>