Changeset 5d3d281 for tests/nowarn

Remove autogen forward declarations, which are never needed, and cause warnings about static declarations without definitions.

Intended to fix the failing test from previous commit.

Autogen forward declarations are never needed because they do not depend on each other, much less with mutual recursion.

Consequences:

• tests/.expect/(5 tests).(3 archs).txt: Accept generated code that lacks autogen forward declarations
• libcfa/src/concurrency/thread.*: Remove unused dependency on destructor from constructor (via thrd_start), by splitting trait is_thread with is_basic_thread