More aggresively move_into_the_past for tests

Right now we're accidentally not actually leveraging this function well in some
tests due to this sequence of events:

1. The tests run at time X, building artifacts that remember the fingerprint is
   at time X.
2. The entire project is moved back one hour to (X-1).
3. A new file is created, at time X (second-level resolution on some systems).
4. On a rebuild, the maximum mtime is still X (due to the new file).

For this reason there are some more calls to move_into_the_past() to push files
back another hour after they've been created to make sure the maximum mtime is
(X-1), or something different than X.
auto merge of #803 : alexcrichton/cargo/fix-some-tests, r=alexcrichton

Right now we're accidentally not actually leveraging this function well in some
tests due to this sequence of events:

1. The tests run at time X, building artifacts that remember the fingerprint is
   at time X.
2. The entire project is moved back one hour to (X-1).
3. A new file is created, at time X (second-level resolution on some systems).
4. On a rebuild, the maximum mtime is still X (due to the new file).

For this reason there are some more calls to move_into_the_past() to push files
back another hour after they've been created to make sure the maximum mtime is
(X-1), or something different than X.