t: adapt test_readlink() to not use Perl

The `test_readlink()` helper function reads a symbolic link and returns
the path it is pointing to. It is thus equivalent to the readlink(1)
utility, which isn't available on all supported platforms. As such, it
is implemented using Perl so that we can use it even on platforms where
the shell utility isn't available.

While using readlink(1) is not an option, what we can do is to implement
the logic ourselves in our test-tool. Do so, which allows a bunch of
tests to pass when Perl is not available.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -1979,7 +1979,7 @@ test_remote_https_urls() {
 # Print the destination of symlink(s) provided as arguments. Basically
 # the same as the readlink command, but it's not available everywhere.
 test_readlink () {
-	perl -le 'print readlink($_) for @ARGV' "$@"
+	test-tool path-utils readlink "$@"
 }
 
 # Set mtime to a fixed "magic" timestamp in mid February 2009, before we