refactor(test/clitools)!: refine semantics of CliTestContext::kill_at()

[rami3l]

Refines #4963 before going further with e.g. #4965.

cc @cachebag

[cachebag]

I reworked #4965 a bit to follow the patterns you have here. I will plan to rebase this onto it once it is merged. One thing maybe to note for myself it to drop my local copy of the metadata checkpoint name in favor ofCHECKPOINT_UPDATE_BEFORE_METADATA.

One question if you don't mind: for testing concurrent installs of the same toolchain, I'll need to keep a child parked at a checkpoint while a second command runs, then kill it explicitly. Would you be open to a spawn_at(checkpoint, args)-style primitive returning a guard (child killed on drop), with kill_at becoming a thin wrapper over it? I can propose that when I rebase.

View changes since this review

[cachebag]

Should this be dropped right after spawn()? Similar to as it already hapens here:

rustup/src/test/clitools.rs

Lines 479 to 481 in 09b779d

	let lock = CMD_LOCK.read().unwrap();
	let out = cmd.output();
	drop(lock);