Fixed running all tests together.

rake test:unit (which runs make check) runs the tests in isolation. That
takes long. (93s)

This enables running them all together which "rspec" alone does by
default. (24s)

Also made tests independent of order they are run in (--order random).

Avoid redefining constants:
- move a constant outside a .each loop
- rename a constant
test helper: respect existing Y2DIR

to enable manual testing, specifying yast2.rpm paths with
`Y2DIR=... rspec ...`