Rollup merge of #48095 - QuietMisdreavus:doctest-assembly, r=GuillaumeGomez

add unit tests for rustdoc's processing of doctests

cc #42018

There's a lot of things that rustdoc will do to massage doctests into something that can be compiled, and a lot of options that can be toggled to affect this. Hopefully this list of tests can show off that functionality.

The first commit is slightly unrelated but doesn't touch public functionality, because i found that if you have a manual `fn main`, it adds an extra line break at the end, whereas it would trim this extra line break if it were putting a `fn main` in automatically. That first commit makes it trim out that whitespace ahead of time.