Build testDowngrade as copy of testUpgrade.

Note that only test program name is changed, some test cases within
testDowngrade still may have upgrade as part of name.  Kept them as
no effort have been put to revise whether individual test cases makes
sense in downgrade or not.

Copy conf-upgrade*.cnf to conf-downgrade*.cnf.

Create downgrade-tests.txt from upgrade-tests.txt replacing testUpgrade with testDowngrade.

In autotest-run.sh also copy missing upgrade/downgrade files to support downgrade testing to old versions not having downgrade files.

Since testUpgrade and testDowngrade links against libndbclient dynamically old api will be tested when changed test program to old version even if program is copied from new version.