Spring Integration's FileWritingMessageHandler uses a .writing file
while it's in the process of writing a message to disk and then
performs a rename (depending on the OS and filesystem this may or may
not be atommic) to create the .msg file. Prior to this commit the
test was finding the temporary .writing files and examining them. This
could lead to a FileNotFoundException being thrown as the temporary
file was deleted while the test was trying to read its contents.

This commit updates the test to only look for files with a .msg suffix

Fixes gh-2428