e2e Scenarios: Refine the gc checking resolution

- Start checking for potential garbage collection a few seconds sooner.
- Check for potential garbage collection many times within each second.
- Generate more changes dirt for gc-executor's jgit client to collect.

These together help compare different jgit versions used in gc-executor.

Change-Id: I2e352421c5229c85bb6f52bf0f2c8e2a9048cdee
2 files changed