In most builds, we can assume that engines live in the build tree
subdirectory "engines".  This was hard coded into the tests that use
the engine ossltest.

However, that hard coding is tedious, it would need to be done in
every test recipe, and it's an incorrect assumption in some cases.

This change has us play it safe and let the build files tell the
testing framework where the engines are.
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>