test: add a test case for override_temp_dir (#289)

Co-authored-by: Sebastian Urban <surban@surban.net>

tests/env.rs

@@ -0,0 +1,15 @@
+#![deny(rust_2018_idioms)]
+
+use std::path::Path;
+
+#[test]
+fn test_override_temp_dir() {
+    assert_eq!(tempfile::env::temp_dir(), std::env::temp_dir());
+
+    let new_tmp = Path::new("/tmp/override");
+    tempfile::env::override_temp_dir(&new_tmp).unwrap();
+    assert_eq!(tempfile::env::temp_dir(), new_tmp);
+
+    let new_tmp2 = Path::new("/tmp/override2");
+    tempfile::env::override_temp_dir(new_tmp2).expect_err("override should only be possible once");
+}