tests: split & speedup datacontainer binary operations

[casperdcl]

• lower tests memory requirements
  • prevents OoM crashes on my laptop
  • also speeds up tests by several seconds
• split into separate test functions
• cleanup/reintroduce timing logic
• misc tidy

👉 diff sans whitespace

[MargaretDuff]

Can we also get rid of the timing steps used for the commented out test on new line 207, 227, 340?

Otherwise looks like simple changes that don't affect the tests but make them quicker and clearer

[MargaretDuff]

Thanks @casperdcl