testkit: add clockStart parameter to TestControl.execute and executeE…

[najuna-brian]

Description

Adds optional clockStart parameter to TestControl.execute and executeEmbed to set initial time offset, making it easier to test time-sensitive code at specific timestamps.

Use Case

This directly addresses the need for testing time-sensitive functionality like HTTP message signatures, authentication tokens, and expiration logic at specific timestamps.

// Before
TestControl.execute(program).flatMap { control =>
  control.tick *> control.advance(timestamp) *> control.tick *> control.results
}

// After  
TestControl.executeEmbed(program, clockStart = timestamp)

Backward compatible with comprehensive test coverage.

Refs #3309

[najuna-brian]

Hi @armanbilge , @samspills the macOS CI failed due to a network timeout while downloading dependencies. Could you please rerun the jobs? Thanks!

[najuna-brian]

Follow-up: I also tried triggering CI with an empty commit, but the Windows job failed with a ServerAlreadyBootingException (SBT lock issue)

[durban]

Backward compatible

No, it's not binary backward compatible, and mima (rightly) complained.