chore: refactor and improve test data structure struct Fragment

[rroelke]

This pull request does some refactoring and enhancing of our test data structures struct Fragment1D and struct Fragment2D which are used as containers for inputs to write queries and outputs of read queries.

1. we move the methods which generically create arrays using these and write fragments using these to a header file instead of sparse_global_order_reader.cc
2. we refactor functionality into a template <typename DimensionTuple, typename AttributeTuple> struct Fragment which is a common base class for Fragment1D, Fragment2D, a new Fragment3D and can be used with an empty dimension tuple for dense fragments
3. we implement various fixes to enable variable-length dimensions
4. some other incidentals

This pull request has been refactored out of #5646 for two reasons:

1. it is all enhancements to test code which may be distracting from the functional components of chore: Add global order min/max per tile to the fragment metadata #5646 ;
2. I anticipate using it to add additional tests to Fix dense fragment domains during global order write with maximum fragment size #5655 . Specifically, using the Fragment with an empty dimension tuple will be very nice for writing tests on dense arrays.

There's no direct testing of this, but since the code was factored out of #5646 all of it was used in testing there. Many components are exercised due to the refactor of the existing tests.

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TYPE: NO_HISTORY
DESC: refactors and enhancements to test data structure struct Fragment