Refactor tests

[cschreib]

This PR does the following:

• The explicit casting constructor of oup::basic_observable_ptr would not move the deleter. This is now fixed.
• A call to oup::dynamic_pointer_cast on a null pointer would not move the deleter. This is now fixed.
• MSVC would report a false positive warning about a noexcept function that can throw. The warning is now silenced.
• Improved error message for over-aligned types in oup::make_observable.
• (internal only) Improved encapsulation of the ptr_and_deleter object.
• (internal only) Refactored the tests; using snatch instead of Catch2.

[codecov-commenter]

Codecov Report

Merging #16 (f82e691) into main (7a16d59) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          317       347   +30     
=========================================
+ Hits           317       347   +30     

Impacted Files	Coverage Δ
include/oup/observable_unique_ptr.hpp	100.00% <100.00%> (ø)

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