Auto merge of rust-lang#148003 - aDotInTheVoid:rustdoc-postcard-compiletest-groundwork, r=jieyouxu

compiletest: pass rustdoc mode as param, rather than implicitly

Spun out of rust-lang#142642

In the future, I want the rustdoc-json test suite to invoke rustdoc twice, once with `--output-format=json`, and once with the (not yet implemented) `--output-format=postcard` flag.

Doing that requires being able to explicitly tell the `.document()` function which format to use, rather then implicitly using json in the rustdoc-json suite, and HTML in all others.

r? `@jieyouxu`

CC `@jalil-salame`

Author: bors