[red-knot] Use unambiguous invalid-syntax-construct for suppression c…

…omment test (#15933)

## Summary

I experimented with [not trimming trailing newlines in code
snippets](#15926 (comment)),
but since came to the conclusion that the current behavior is better
because otherwise, there is no way to write snippets without a trailing
newline at all. And when you copy the code from a Markdown snippet in
GitHub, you also don't get a trailing newline.

I was surprised to see some test failures when I played with this
though, and decided to make this test independent from this
implementation detail.

crates/red_knot_python_semantic/resources/mdtest/suppressions/knot_ignore.md

@@ -77,7 +77,8 @@ def test(a: f"f-string type annotation", b: b"byte-string-type-annotation"): ...
 ```py
 # error: [invalid-syntax]
 # error: [unused-ignore-comment]
-def test(  # knot: ignore
+def test($):  # knot: ignore
+    pass
 ```
 
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