Stabilize FileIOTest updated-file timestamp assertions

Author: bvolpato

sdks/java/core/src/test/java/org/apache/beam/sdk/io/FileIOTest.java

@@ -257,10 +257,17 @@ public void processElement(ProcessContext context, @StateId("count") ValueState<
 
       if (0 == current) {
         Thread.sleep(100);
+        // Ensure overwrite updates get a distinct mtime even when COPY_ATTRIBUTES is enabled.
+        Files.setLastModifiedTime(
+            sourcePath.resolve("first"), FileTime.fromMillis(System.currentTimeMillis() + 2000));
         Files.copy(sourcePath.resolve("first"), watchPath.resolve("first"), updOptions);
         Files.copy(sourcePath.resolve("second"), watchPath.resolve("second"), cpOptions);
       } else if (1 == current) {
         Thread.sleep(100);
+        Files.setLastModifiedTime(
+            sourcePath.resolve("first"), FileTime.fromMillis(System.currentTimeMillis() + 4000));
+        Files.setLastModifiedTime(
+            sourcePath.resolve("second"), FileTime.fromMillis(System.currentTimeMillis() + 4000));
         Files.copy(sourcePath.resolve("first"), watchPath.resolve("first"), updOptions);
         Files.copy(sourcePath.resolve("second"), watchPath.resolve("second"), updOptions);
         Files.copy(sourcePath.resolve("third"), watchPath.resolve("third"), cpOptions);