check units early for link problems

[westram]

To avoid problems while placing code into libraries, checking library linking is required more early during development.

Currently this requirement is provided by tested units, but this has issues:

• only those objects containing test methods (or code used by these) will be picked by linker
• since [19613], the unit tester also tries to ​check linking for untested units, but this does only work for one ​"randomly" picked symbol resp. its object.
• all other objects in each library remain unchecked (i.e. unsatisfied symbols in those objects will not be reported early).
  These are at first reported when linking some executable, which is considered too late.

Needed fixes:

• link whole testable unit to detect all link problems