Fix test race condition

E-tag propagation replies on the mtime of the file. Order of events:

1. add file 'foo.txt' with content 'bar'
2. Set mtime to now() - 1
3. Check if etag changed.

Now this goes right often when 1 and 2 happen in the same second.
However imagine

1. add file 'foo.txt' with content 'bar' (at t=0.999)
2. Set mtime to now() - 1 (at t=1.001)

Now the mtime will be set to the same time. Thus not chaning the etag.