Cleanup was a best-effort `let _ = std::fs::remove_file(&file)` at the
end of each test, so a panic or timeout left the fixture untracked in
`ck-lsp/tests/`. Replace it with a `FixtureFile` RAII guard whose Drop
removes the file on every exit path — including unwinding from a
failed assertion.

@@ -116,0 +131,4 @@
+}
+
+impl Drop for FixtureFile {
+    fn drop(&mut self) {