Fix flaky write_with_sse_kms_key_id_ok test (#1140)

Fixes the flakiness of write_with_sse_kms_key_id_ok test, which was
caused by not properly unmounting and dropping child.

This is not a breaking change; no changelog entry required (as this just
fixes a test).


Before this change, this test fails in ~10 out of 100 runs, after this
change it fails 0 times out of 100 runs.


---

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and I agree to the terms of
the [Developer Certificate of Origin
(DCO)](https://developercertificate.org/).

Signed-off-by: Christian Hagemeier <chagem@amazon.com>

mountpoint-s3/tests/fuse_tests/fork_test.rs

@@ -828,10 +828,11 @@ fn write_with_sse_kms_key_id_ok() {
     assert!(kms_key_arn.starts_with("arn:") && kms_key_arn.contains(":key"));
     let (bucket, prefix) = get_test_bucket_and_prefix("write_with_sse_kms_key_id_ok");
     let mount_point = assert_fs::TempDir::new().expect("can not create a mount dir");
-    let _ = mount_with_sse(&bucket, mount_point.path(), &prefix, &kms_key_arn, None);
+    let child = mount_with_sse(&bucket, mount_point.path(), &prefix, &kms_key_arn, None);
     let f_name = "f.txt";
     write_to_file(mount_point.path(), f_name).expect("should be able to write to the file");
     unmount(&mount_point);
+    wait_for_exit(child);
 
     let sdk_client = tokio_block_on(get_test_sdk_client(get_test_region().as_str()));
     let key = format!("{}{}", prefix, f_name);