Crater seems to not check whether doctests compile or not

[theemathas]

In order to check whether tests compile, crater runs the command: cargo +some_channel test --frozen --no-run --message-format=json. This compiles unit tests, but don't run them. However, this command does not compile doctests at all.

This means that crater runs cannot detect when doctests stop compiling, which seems like an undesirable blind spot.

See also rust-lang/rust#87022 and rust-lang/cargo#6424.

Discovered while investigating rust-lang/rust#142424.