Build on #18360 which added a way to define relative start timestamp (ST) per loaded sample (in next line).

• Update all tests to use XOR2 chunk with ST and get rid of module init function promqltest: support start timestamp (ST) per sample in load command #18360 (comment)
• Add a way to do cumulative counter constant start timestamp easier. Either by being able to repeat previous ST (in repetition and individual ST) or by allowing absolute timestamps as well.
• Document the feature.