Split compilation of a private helper library

This is needed so that we can link unit tests for these internal classes
Link completion-model test to the private lib

Use the new private lib and remove the workarounds
A small regex unit test
Reenable test-region test

We can use the priv lib to reenable the textregion test and turn it into
a unit test
test-completion-model: more unit tests

The number of functions tested is now in the green. The number of lines
tested is still in orange because of the commented tests.