File-split-ftp: fix read files race condition

https://build.spring.io/browse/INTSAMPLES-NIGHTLY-1898/

There is no guarantee that if `File.exists()`, the real write to file has occurred

 * Add `while()` to check not only `File.exists()` but also its `length()` only after that proceed to assert logic

applications/file-split-ftp/src/test/java/org/springframework/integration/samples/filesplit/ApplicationTests.java

@@ -118,12 +118,20 @@ private String runTest(boolean fail) throws Exception {
 		br.close();
 		out.delete();
 		out = new File("/tmp/out/006.txt");
+		n = 0;
+		while(n++ < 100 && (!out.exists() || out.length() < 12)) {
+			Thread.sleep(100);
+		}
 		assertThat(out.exists()).isTrue();
 		br = new BufferedReader(new FileReader(out));
 		assertThat(br.readLine()).isEqualTo("*006,baz,qux");
 		br.close();
 		out.delete();
 		out = new File("/tmp/out/009.txt");
+		n = 0;
+		while(n++ < 100 && (!out.exists() || out.length() < 12)) {
+			Thread.sleep(100);
+		}
 		assertThat(out.exists()).isTrue();
 		br = new BufferedReader(new FileReader(out));
 		assertThat(br.readLine()).isEqualTo("*009,fiz,buz");