Tests: the parity-reach fixture names files the way the tool does

narrow() and its graph key files by Posix path (`crates/a/src/used.rs`), so
a fixture built from str(relative_to) on Windows produced backslashes and all
seven tests errored in setUp. as_posix() in the fixture matches the tool.

Co-Authored-By: Hermes Agent (Nous Research) <noreply@nousresearch.com>

tests/test_parity_reach.py

@@ -60,7 +60,7 @@ class ReachTests(unittest.TestCase):
                 pub fn main() { println!("{} {} {}", helper(), Thing.method(), Thing); }
             """,
         })
-        self.dependencies = sorted(str(path.relative_to(self.root)) for path in self.root.rglob("*") if path.is_file())
+        self.dependencies = sorted(path.relative_to(self.root).as_posix() for path in self.root.rglob("*") if path.is_file())
         self.dependencies.remove("crates/a/examples/probe/main.rs")  # parity.only_probes_run leaves it out
 
     def tearDown(self):