Add some tests about S3 timestamp tracking

We need to make sure we understand timestamp behaviour across S3 implementations, for both single part and multipart uploads

tests to verify that the timestamp of files is consistent

• individual files's timestamps don't appear to change
• if you upload two small files in sequence, the timestamp of file 2> file 1

MPU uploads are different: their timestamp may be that of the upload start, rater than upload delete

A sequence of

1. init MPU to file1
2. PUT to file 2
3. complete MPU to file1

may result in timestamp(file1) < timestamp(file2)

but if you add a PUT to file 0 before step 1, then we require that timestamp(file0) < timestamp(file1). That is: there's still an ordering on initialization.

these tests are probably relevant to all MPU implementations, though they'll offer different timing outcomes