Tests to exercise combinations of views for external sort. (#5051)

This PR provides the file `tiledb/common/util/test/unit_view_combo.cc`
that includes a set of tests of various combinations of the view classes
that have been developed for external sort. In particular, we have tests
to `chunk` a `chunk_view`, `var_length_view`, and `alt_var_length_view`,
to `chunk` a `permutation_view` and permute a `chunk_view`, and to
`chunk` a `zip_view` and `zip` a `chunk_view`.

The tests effect these compositions by defining a range, taking a view
of it (by calling, e.g., `chunk()`), and then calling another view
adaptor. We do not intend to support composition by `operator|()` at
this time.

[sc-48753]

---
TYPE: NO_HISTORY
DESC: Tests to exercise combinations of views.

---------

Co-authored-by: Luc Rancourt <[email protected]>
Co-authored-by: KiterLuc <[email protected]>

Author: lums658

tiledb/common/util/permutation_view.h

@@ -143,13 +143,13 @@ class permutation_view : public std::ranges::view_interface<
         data_begin_, index_begin_, data_end_ - data_begin_};
   }
 
-  /** Size of the permutation view */
+  /** Size of the permutation view. */
   size_t size() const {
     return data_end_ - data_begin_;
   }
 
-  /** Accessor */
-  auto& operator[](size_t i) const {
+  /** Accessor.  Note use of decltype(auto).  */
+  decltype(auto) operator[](size_t i) const {
     // More general? return *(data_begin_ + *(index_begin_ + i));
     return data_begin_[index_begin_[i]];
   }

tiledb/common/util/test/CMakeLists.txt

@@ -74,3 +74,8 @@ commence(unit_test block_view)
 )
 conclude(unit_test)
 
+commence(unit_test view_combo)
+this_target_sources(
+        unit_view_combo.cc
+)
+conclude(unit_test)