[3.12] gh-108418: Speed up bigmem compression tests in dry mode (GH-108419) (#108473)

gh-108418: Speed up bigmem compression tests in dry mode (GH-108419)

Only generate and compress small amount of random data in dry run.
(cherry picked from commit 4ae3edf3008b70e20663143553a736d80ff3a501)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>