Unify keyfile-to-configuration conversions in system tests

The following code snippet is repeated (with slight variations) 29 times throughout bin/tests/system/:

cat $keyname.key | grep -v '^; ' | $PERL -n -e '
local ($dn, $class, $type, $flags, $proto, $alg, @rest) = split;
local $key = join("", @rest);
print <<EOF
trusted-keys {
    "$dn" $flags $proto $alg "$key";
};
EOF
' > trusted.conf

To reduce code duplication and make future changes easier, this snippet should be replaced with a helper function in bin/tests/system/conf.sh.{in,win32}.