run-make-support: use a `CommandWrapper` trait instead of `impl_helper_methods` macro

[jieyouxu]

impl_helper_methods worked reasonably well while prototyping, but now I realized a trait with default provided methods is a better fit for making it easier and less verbose to implement command wrappers. This is because command_output can be a required method checked by rustc, while we can provide helper methods but still allow individual command wrapper instances to override as desired.

pub trait CommandWrapper {
    // required method
    fn command_output(self) -> std::command::Output;

    // provided helper methods
    fn run(self) -> std::command::Output { ... }
    fn run_fail(self, expected_exit_code: i32) -> std::command::Output { ... }
}

then implementors can write

pub struct Rustc { cmd: Command }

impl CommandWrapper for Rustc {
    fn command_output(self) -> Output { ... }
}

impl Rustc {
    // ... other helper methods
}

Unresolved questions

• If these helper methods are moved on to a trait, then test writers will have to bring the trait in scope in order to use the helper methods. Is it worth the extra typing?

[jieyouxu]

Drawback: test writer would need to bring the trait into scope to use the helper methods...

[ChrisDenton]

There could be inherent methods that simply forward to the trait methods. That's more boilerplate though.

[Kobzol]

We don't need command_output on each wrapper struct anymore as of #126121. I don't think that a trait pulls its weight here, due the tests having to import it (it's a bit annoying since run-make tests are typically not very friendly with IDEs). It would be nice to remove the macro, but we can do that also with other approaches than a trait (e.g. Deref).