test(script): Switch frontmatter tests to end-to-end (#15899)

### What does this PR try to resolve?

For frontmatter parser, we almost exclusively use unit tests.

This switches the rustc ui tests we've copied over to being cargo ui
tests but in a way that makes it relatively easy to diff against rustc
and see the state of the two and makes it easier to update.

This will help as I work to improve the errors reported by cargo to
align with rustc in quality

As part of this, I made the source file auto-updating whenever new tests
get copied over.

### How to test and review this PR?

### Notes

Note: the tests are a little ahead of rustc atm, being pulled from one
of my branches that I've not yet pushed because its blocked on another
PR

Author: weihanglo

src/cargo/util/toml/embedded.rs

@@ -0,0 +1,2 @@
+mod cargo;
+mod rustc;