build(gha): Reduce backend test instances, change acceptance grouping (#21887)
* Reduce backend tests instances from 3 -> 2
* Our slowest tests take around 18m, where as these backend tests take around 10m, to be more efficient with concurrent test runs we want tests to have similar durations.
* Change the acceptance test grouping strategy to be per test case instead of per file. This ensures (for now) a better balancing of our tests. Otherwise, there was one test instance that ran our 2 slowest test suites (issue details and eventsv2) and would always finish 4-5 minutes after the fastest acceptance suite. With round robin, we distribute tests between all 3 instances.
* Previous acceptance test runs per instance would roughly take around 14m, 18m, and 16m
Billy Vong