chore: refactor and improve test data structure struct Fragment (#5675)

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 #5646 ;
2) I anticipate using it to add additional tests to #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`

Author: rroelke