Changes

gbproxy: Fix several tests on titan 11

Rework altsteps to avoid race condition showing up under titan 11.1.0.
It's not really clear whether the previous implementation is actually
expected/permitted by TTCN-3, where an altstep variable is initialized
multiple times with different values through activate().
In any case, the new implementation is much cleaner, only requiring 1
altstep instead of N.

Related: OS#6800
Change-Id: Ib17c7dcb3d224a6b9e75ce5f9121a05126f81909