tests: multipart: use REDUCED_PART_SIZE and variable part size
tests: multipart: use REDUCED_PART_SIZE and variable part size

There is no reason to use 5M chunks, especially with the reduced_part_size
decorator. Also made part_size to be variable to add extra layer of
testing to make sure that moto is handling that scenario correctly.

Signed-off-by: Ruslan Kuprieiev <e9bfb2c543dd33934bcff71879b108246996b567@iterative.ai>

william-richard/moto,william-richard/moto,william-richard/moto,whummer/moto,whummer/moto,spulec/moto,william-richard/moto,spulec/moto,spulec/moto,spulec/moto,spulec/moto,william-richard/moto,whummer/moto,whummer/moto,whummer/moto,whummer/moto,william-richard/moto,spulec/moto