[red-knot] Port type inference tests to new test framework (#13719)

## Summary

Porting infer tests to new markdown tests framework.

Link to the corresponding issue: #13696

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/declaration/error.md

@@ -0,0 +1,39 @@
+# Errors while declaring
+
+## Violates previous assignment
+
+```py
+x = 1
+x: str  # error: [invalid-declaration] "Cannot declare type `str` for inferred type `Literal[1]`"
+```
+
+## Incompatible declarations
+
+```py
+if flag:
+    x: str
+else:
+    x: int
+x = 1  # error: [conflicting-declarations] "Conflicting declared types for `x`: str, int"
+```
+
+## Partial declarations
+
+```py
+if flag:
+    x: int
+x = 1  # error: [conflicting-declarations] "Conflicting declared types for `x`: Unknown, int"
+```
+
+## Incompatible declarations with bad assignment
+
+```py
+if flag:
+    x: str
+else:
+    x: int
+
+# error: [conflicting-declarations]
+# error: [invalid-assignment]
+x = b'foo'
+```