add test for no constraint scenario

an important test of the alignment procedure is to run without any constraints, which should result in all eigenvalues being zero. Ideally, one would have one tracker alignable with translations and rotations.

No constraints means:

• no hits in external detectors
• no survey constraints
• no Lagrange constraints
• no constraints from initial K-matrix

cc @wouter

see alo the discussion in !228 (merged):

• @wouter started a discussion: (+40 comments)

  There is one validation step that we need, which is probably not part of an automated test. If the alignment derivatives and trackfit are correctly implemented, then if we remove all 'external constraints' (e.g. survey constraints), then we should find exactly 6 zero eigenvalues. This test was very important to debug the original code. Somehow we need to revive it.