Modularize interpolation testing

[KnutAM]

This PR includes some modularization of the testing of interpolations started in #798, and is the first breakout from that PR.

Specifically all tests that every base interpolations should pass should be put into test_interpolation_properties(ip::Interpolation). Having such a function was very valuable when developing the interpolations in #798, and it also makes it easier to include it in testing of such exotic interpolations when all assumptions currently tested for scalar and nodal interpolations don't hold.

It also separates testing of the reference shapes from the testing of the interpolations, and fixes one discovered issue with the thrown error from reference_shape_value(ip::DiscontinuousLagrange{shape, 0}, ... (it silently accepted i > n_basefunctions)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.30%. Comparing base (951ec47) to head (8d1807a).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1136      +/-   ##
==========================================
- Coverage   93.57%   93.30%   -0.28%     
==========================================
  Files          39       39              
  Lines        6073     6106      +33     
==========================================
+ Hits         5683     5697      +14     
- Misses        390      409      +19     

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