ci/windows: Bump the number of tests per group and add a nightly full job

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 allows us to run more tests and/or reduce runtime. Initially, I attempted to use an even lower DEQP_FRACTION, but reruns of a stress test showed increasingly lower performance, likely due to the farm struggling with the higher load. A fraction of 4 seemed to be a reliable and safe choice.

Additionally, add a new manual full job. This nightly run takes approximately 15 minutes to complete.