venus,lavapipe/ci: Bump the number of tests per group

Due to the slow startup time of deqp-vk, the previous default of 500 tests per group caused the jobs to run up to twice as slowly compared to using a higher number of tests per group.

Increasing tests_per_group enables us to run more tests and/or decrease runtime.

For venus-lavapipe, I didn't lower the fraction because the job was consistently taking longer than the 10 minute target. With these speedups, both the venus-lavapipe and lavapipe jobs are taking 8-9 minutes.

There's a lot of flakes added for lavapipe, but these are not related to the increased tests_per_group. The job was already quite flaky, as seen in various older pipelines: 1 2 3