Suppress system Term::ReadLine::Gnu

Suppress system Term::ReadLine::Gnu

perl5db.t will die in Term::ReadLine if Term::ReadLine::Gnu is installed
in the system. Let's favour core implementation.

Best solution would be to prune @INC to prevent from loading already
installed modules like this:

BEGIN {
    use Config;
    @INC = grep { ! /^\Q$Config{installprefix}\E(\/|\z)/ } @INC;
}

However that is not possible (now) due to various execs (even without
proper -I) in the harness chain. perl should implement -nostdinc
option.

Signed-off-by: Petr Písař <ppisar@redhat.com>