[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/subscript/string.md

@@ -0,0 +1,32 @@
+# Subscript on strings
+
+## Simple
+
+```py
+s = 'abcde'
+
+a = s[0]
+b = s[1]
+c = s[-1]
+d = s[-2]
+e = s[8]        # error: [index-out-of-bounds] "Index 8 is out of bounds for string `Literal["abcde"]` with length 5"
+f = s[-8]       # error: [index-out-of-bounds] "Index -8 is out of bounds for string `Literal["abcde"]` with length 5"
+
+reveal_type(a)  # revealed: Literal["a"]
+reveal_type(b)  # revealed: Literal["b"]
+reveal_type(c)  # revealed: Literal["e"]
+reveal_type(d)  # revealed: Literal["d"]
+reveal_type(e)  # revealed: Unknown
+reveal_type(f)  # revealed: Unknown
+```
+
+## Function return
+
+```py
+def add(x: int, y: int) -> int:
+    return x + y
+
+a = 'abcde'[add(0, 1)]
+# TODO: Support overloads... Should be `str`
+reveal_type(a)  # revealed: @Todo
+```