Centralise Environment Variables for the tests

Centralise Environment Variables for the tests

The test_includes test was failing if OPENSSL_CONF_INCLUDE happened to
be set in the user's environment. To ensure that no tests accidentally
use this or other enviroment variables from the user's environment we
automatically set them centrally for all tests.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11691)