Re #8358. Clear memory leaks in test.

There's no reason for any of these MaskWorkspace instances to be created
on the heap rather than the stack.
Re #8358. Eliminate memory leak by creating object on stack.
Re #8358. Eliminate a raft of memory leaks in this test.
Re #8358. Eliminate memory leaks in test.
Re #8358. Eliminate memory leaks in test.
Merge branch 'bugfix/8358_dataobjects_test_memory_leaks' into develop