Add an alias to build test images simply

[SomeoneToIgnore]

Follow-up of #2446

Yet, this makes local testing harder since all failpoints need a special flag to be used during cargo build to work locally.
rust-lang/cargo#4829 hints it's impossible to set cargo features via env vars, so there seems to be no good way to get around it with cargo config file.

fixing that, by adding a new "command" build_testing that unfolds to build --features failpoints now.

I think build_* name should reflect the name of the feature we select for #2429 (comment) idea

(a) I think that if we're using a feature flag to turn on/off testing APIs, it should be something more general than failpoints, and (b) I'm not sure what the name for that should be, or if there's a name that's typically used.

I've picked testing as a proposal, but feel free to propose yours.

[sharnoff]

Looks good - I like the name testing. I think it makes the most sense for me to rebase #2429 on top of this and add the testing feature there (or testing-api, whatever we choose).

[SomeoneToIgnore]

I think, there comes more than API with that, so just testing- seems ok, unless better options appear.