roachtest: fix perturbation/metamorphic/backfill

[andrewbaptist]

Previously this test was setting the memory spec before it called randomize. This could set it back to low memory. We already know that low memory configuration doesn't work for backfill as part of #133114.

This change calls randomize and then sets the memory configuration.

Epic: none Fixes: #135799

Release note: None

[cockroach-teamcity]

This change is 

[andrewbaptist]

TFTR

bors r=kvoli

[craig[bot]]

Build succeeded:

• macos_amd64_build
• acceptance
• examples_orms
• local_roachtest_fips
• windows_build
• lint
• linux_amd64_build
• linux_amd64_fips_build
• macos_arm64_build
• check_generated_code
• linux_arm64_build
• unit_tests
• local_roachtest
• docker_image_amd64