Set different env vars for each test in debug.rs

This is important because test run in parallel,
if both tests are changing the same env var at the
same time, we get intermittent failures.

glean-core/src/debug.rs

@@ -171,8 +171,8 @@ mod test {
 
     #[test]
     fn debug_option_is_correctly_loaded_from_env() {
-        env::set_var("GLEAN_TEST", "test");
-        let option: DebugOption<String> = DebugOption::new("GLEAN_TEST", None);
+        env::set_var("GLEAN_TEST_1", "test");
+        let option: DebugOption<String> = DebugOption::new("GLEAN_TEST_1", None);
         assert_eq!(option.get().unwrap(), "test");
     }
 
@@ -187,8 +187,8 @@ mod test {
         }
 
         // Invalid values from the env are not set
-        env::set_var("GLEAN_TEST", "invalid");
-        let mut option: DebugOption<String> = DebugOption::new("GLEAN_TEST", Some(validate));
+        env::set_var("GLEAN_TEST_2", "invalid");
+        let mut option: DebugOption<String> = DebugOption::new("GLEAN_TEST_2", Some(validate));
         assert!(option.get().is_none());
 
         // Valid values are set using the `set` function