So that it can be also used in bootstrap.

To enable the previously moved `DropBomb` tests.

Before, only the line was stored. This was enough for run-make tests, since these mostly only contain a single `rmake.rs` file, but not for bootstrap.

… it through the `as_command_mut` method

This will be useful for disarming drop bombs when the inner command is accessed.

This makes it harder to accidentally forget to execute a created command in bootstrap.

This should make it quicker to debug command failures.

Avoid printing useless information in the `Debug` output.

We can move the command creation to a block where it is clear that the command will be executed.

The code for running tests uses a custom command machinery because it streams the output of the command. We thus need to mark the command as executed in a dry run, to avoid a drop bomb panic.

If we're in dry run mode, the command will return an empty string, so we can just execute it.

There is no need to build a stage N toolchain for testing it.