PLT-8314: Test Message Export Against S3 Bucket (#7957)

* Removed export directory config setting, in favour of hard-coding it to an 'export' directory under the local file directory. Improved the local file backend copy implementation to implicitly create the destination directory if it's missing

* Fixed the tests

utils/file.go

@@ -21,6 +21,9 @@ func CopyFile(src, dst string) (err error) {
 	}
 	defer in.Close()
 
+	if err = os.MkdirAll(filepath.Dir(dst), os.ModePerm); err != nil {
+		return
+	}
 	out, err := os.Create(dst)
 	if err != nil {
 		return