test(sdk-core): Use Flowable.just instead of hand-written mock publisher in FileAsyncResponseTransformerPublisherTest to prevent flaky test failures (#7044)

Author: joviegas

core/sdk-core/src/test/java/software/amazon/awssdk/core/internal/async/FileAsyncResponseTransformerPublisherTest.java

@@ -22,6 +22,7 @@
 import static org.mockito.Mockito.when;
 
 import com.google.common.jimfs.Jimfs;
+import io.reactivex.Flowable;
 import java.nio.ByteBuffer;
 import java.nio.file.FileSystem;
 import java.nio.file.Files;
@@ -139,18 +140,7 @@ private void createFileIfNeeded(AsyncResponseTransformer<SdkResponse, SdkRespons
     }
 
     private SdkPublisher<ByteBuffer> createMockPublisher() {
-        return s -> s.onSubscribe(new Subscription() {
-            @Override
-            public void request(long n) {
-                s.onNext(ByteBuffer.wrap("test data".getBytes()));
-                s.onComplete();
-            }
-
-            @Override
-            public void cancel() {
-                // unused for tests
-            }
-        });
+        return SdkPublisher.adapt(Flowable.just(ByteBuffer.wrap("test data".getBytes())));
     }
 
     @ParameterizedTest
@@ -256,17 +246,7 @@ private SdkResponse createMockResponseWithRange(String contentRange, Long conten
     }
 
     private SdkPublisher<ByteBuffer> createMockPublisherWithData(byte[] data) {
-        return s -> s.onSubscribe(new Subscription() {
-            @Override
-            public void request(long n) {
-                s.onNext(ByteBuffer.wrap(data));
-                s.onComplete();
-            }
-
-            @Override
-            public void cancel() {
-            }
-        });
+        return SdkPublisher.adapt(Flowable.just(ByteBuffer.wrap(data)));
     }
 
     private static Stream<Function<Path, AsyncResponseTransformer<SdkResponse, SdkResponse>>> transformers() {