Split collection_mocked.cpp test case into smaller functions

[eramongodb]

Related to CXX-3180. mingw-w64 chokes on when attempting to compile the collection_mocked.cpp component:

as.exe: src/mongocxx/
test/CMakeFiles/test_driver.dir/Debug/v_noabi/collection_mocked.cpp.obj: too many sections (37098)
Z:\data\tmp\ccuJcb3j.s: Assembler messages:
Z:\data\tmp\ccuJcb3j.s: Fatal error: can't write 183 bytes to section .text of src/mongocxx/test/CMakeFiles/test_driver.dir/Debug/v_noabi/collection_mocked
.cpp.obj: 'file too big'

The single TEST_CASE() in collection_mocked.cpp is huge (~1K lines). This PR splits it into smaller test cases. Section descriptions (e.g. SECTION(name, description)) are removed due to providing dubious value (possibly due to author confusion?).

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