TST,MAINT: DLPack could use some additional tests and cleanups #20339

DLpack could probably use some extra tests, and followups as noted here: #19083 (comment)

Some of these are:

• Exporting ndim=0 is OK, but may not be tested
• DLpack could do a single allocation for both the struct and strides (small optimization)
• Test for calling _from_dlpack twice (i.e. test the renaming mechanism)