Various tests fixes

* Use a tmp file in the `FileTests.testFileReadingFlow()`
for writing content and then rename it to the target file.
It looks like we may already have a file just with a short content
and it is picked up by the watch service for processing in the flow
* Rework `AggregatorWithCustomReleaseStrategyTests` to JUnit 5 and remove
extra looping logic since it just does not add any extra coverage just
performance overhead via crating and destroying the same application ctx
* Rework `ChannelAdapterParserTests` to `@SpringJUnitConfig` managed test

spring-integration-file/src/test/java/org/springframework/integration/file/dsl/FileTests.java

@@ -201,10 +201,12 @@ public class FileTests {
 			if (even) {
 				evens.add(i);
 			}
-			FileOutputStream file = new FileOutputStream(new File(tmpDir, i + extension));
-			file.write(("" + i).getBytes());
-			file.flush();
-			file.close();
+			File tmpFile = new File(tmpDir, i + extension + ".tmp");
+			FileOutputStream stream = new FileOutputStream(tmpFile);
+			stream.write(("" + i).getBytes());
+			stream.flush();
+			stream.close();
+			tmpFile.renameTo(new File(tmpDir, i + extension));
 		}
 
 		Message<?> message = fileReadingResultChannel.receive(60000);