pythongh-102781: fix cwd dependence in cases generator (python#102782)

Author: carljm

Tools/cases_generator/generate_cases.py

@@ -122,8 +122,9 @@ def __init__(
         self.prefix = " " * indent
         self.emit_line_directives = emit_line_directives
         self.lineno = 1
+        filename = os.path.relpath(self.stream.name, ROOT)
         # Make filename more user-friendly and less platform-specific
-        filename = self.stream.name.replace("\\", "/")
+        filename = filename.replace("\\", "/")
         if filename.startswith("./"):
             filename = filename[2:]
         if filename.endswith(".new"):
@@ -567,6 +568,7 @@ def parse_file(self, filename: str, instrs_idx: dict[str, int]) -> None:
         with open(filename) as file:
             src = file.read()
 
+        filename = os.path.relpath(filename, ROOT)
         # Make filename more user-friendly and less platform-specific
         filename = filename.replace("\\", "/")
         if filename.startswith("./"):