OpenSSL::Test: bring back the relative paths

OpenSSL::Test: bring back the relative paths

Because there was a bug in File::Spec::Unix' abs2rel when it was given
relative paths as both PATH and BASE arguments, the directories we
deal with were made to be all absolute.  Unfortunately, this meant
getting paths in our verbose test output which are difficult to use
anywhere else (such as a separate test build made for comparison), due
to the constant need to edit all the paths all the time.

We're therefore getting back the relative paths, by doing an extra
abs2rel() in __srctop_file, __srctop_dir, __bldtop_file and
__bldtop_dir, with a 'Cwd::getcwd' call as BASE argument.

Fixes #10628

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10913)