rust: try to work around windows permission problem during testing

rust/tests/model.rs

@@ -1,5 +1,5 @@
 mod common;
-use std::{f64::consts::PI, ffi::CString, fs::remove_file};
+use std::{f64::consts::PI, ffi::CString, fs::remove_file, fs::rename};
 
 use common::{get_model, model_dir};
 
@@ -50,7 +50,14 @@ fn model_compiling() {
     base.push(name);
     let lib_path = base.join(format!("{}_model.so", name));
     let stan_path = base.join(format!("{}.stan", name));
-    remove_file(lib_path).unwrap();
+    // try to remove .so file, otherwise try to rename it
+    remove_file(lib_path.as_path()).unwrap_or_else(|_| {
+        rename(
+            lib_path.as_path(),
+            lib_path.to_str().unwrap().to_owned() + "_old",
+        )
+        .unwrap()
+    });
     compile_model(stan_path, vec![], vec![], None).unwrap();
 
     let (lib, data) = get_model(name);