[red-knot] Litate tests: minor follow-up (#15987)

## Summary

- Minor wording update
- Code improvement (thanks Alex)
- Removed all unnecessary filenames throughout our Markdown tests (two
new ones were added in the meantime)
- Minor rewording of the statically-known-branches introduction

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -393,8 +393,6 @@ reveal_type(D().x)  # revealed: Unknown | Literal[1]
 
 If `staticmethod` is something else, that should not influence the behavior:
 
-`other.py`:
-
 ```py
 def staticmethod(f):
     return f
@@ -409,8 +407,6 @@ reveal_type(C().x)  # revealed: Unknown | Literal[1]
 
 And if `staticmethod` is fully qualified, that should also be recognized:
 
-`fully_qualified.py`:
-
 ```py
 import builtins
 

crates/red_knot_python_semantic/resources/mdtest/statically_known_branches.md

@@ -7,43 +7,36 @@ branches whose conditions we can statically determine to be always true or alway
 useful for `sys.version_info` branches, which can make new features available based on the Python
 version:
 
-`module1.py`:
+If we can statically determine that the condition is always true, then we can also understand that
+`SomeFeature` is always bound, without raising any errors:
 
 ```py
 import sys
 
-if sys.version_info >= (3, 9):
-    SomeFeature: str = "available"
-```
+class C:
+    if sys.version_info >= (3, 9):
+        SomeFeature: str = "available"
 
-If we can statically determine that the condition is always true, then we can also understand that
-`SomeFeature` is always bound, without raising any errors:
-
-`test1.py`:
-
-```py
-from module1 import SomeFeature
-
-# SomeFeature is unconditionally available here, because we are on Python 3.9 or newer:
-reveal_type(SomeFeature)  # revealed: str
+# C.SomeFeature is unconditionally available here, because we are on Python 3.9 or newer:
+reveal_type(C.SomeFeature)  # revealed: str
 ```
 
 Another scenario where this is useful is for `typing.TYPE_CHECKING` branches, which are often used
 for conditional imports:
 
-`module2.py`:
+`module.py`:
 
 ```py
 class SomeType: ...
 ```
 
-`test2.py`:
+`main.py`:
 
 ```py
 import typing
 
 if typing.TYPE_CHECKING:
-    from module2 import SomeType
+    from module import SomeType
 
 # `SomeType` is unconditionally available here for type checkers:
 def f(s: SomeType) -> None: ...