t: adapt `test_readlink()` to not use Perl

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>