Add method to config that uses an empty test environment (#3555)

## Motivation and Context
Improve testability

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: Zelda Hessler <[email protected]>

aws/rust-runtime/aws-config/src/lib.rs

@@ -519,6 +519,16 @@ mod loader {
             ret
         }
 
+        /// Ignore any environment variables on the host during config resolution
+        ///
+        /// This allows for testing in a reproducible environment that ensures any
+        /// environment variables from the host do not influence environment variable
+        /// resolution.
+        pub fn empty_test_environment(mut self) -> Self {
+            self.env = Some(Env::from_slice(&[]));
+            self
+        }
+
         /// Override the access token provider used to build [`SdkConfig`].
         ///
         /// # Examples
@@ -782,6 +792,7 @@ mod loader {
                     .region()
                     .await
             };
+            let conf = conf.with_region(region.clone());
 
             let retry_config = if let Some(retry_config) = self.retry_config {
                 retry_config