[red-knot] mdtest suite: formatting and cleanup (#13806)

Minor cleanup and consistent formatting of the Markdown-based tests.

- Removed lots of unnecessary `a`, `b`, `c`, … variables.
- Moved test assertions (`# revealed:` comments) closer to the tested
object.
- Always separate `# revealed` and `# error` comments from the code by
two spaces, according to the discussion
[here](https://github.com/astral-sh/ruff/pull/13746/files#r1799385758).
This trades readability for consistency in some cases.
- Fixed some headings

crates/red_knot_python_semantic/resources/mdtest/assignment/annotations.md

@@ -13,7 +13,7 @@ reveal_type(y)  # revealed: Literal[1]
 ## Violates own annotation
 
 ```py
-x: int = 'foo' # error: [invalid-assignment] "Object of type `Literal["foo"]` is not assignable to `int`"
+x: int = 'foo'  # error: [invalid-assignment] "Object of type `Literal["foo"]` is not assignable to `int`"
 
 ```