Missing unit tests

[mpusz]

I had to speed up refactoring, design, and implementation of the V2 design, and because of it plenty of unit tests are missing now.

Any help would be appreciated.

Here is the list of features not covered with unit tests:

• [ ] DimensionOf and QuantitySpecOf in concept_test.cpp
• [ ] Magnitude tests
• [ ] quantity_point tests for equality, ordering, common_type, value_cast, quantity_cast, in, force_in, derived quantity_point
• [ ] interoperability with other libraries (https://github.com/mpusz/mp-units/pull/495)
• [ ] multiplication and division of quantities by units (https://github.com/mpusz/mp-units/pull/496)