Fix the ghci063 test on Darwin (Trac Trac #16201)
We use "touch -r" to set modification timestamps, which leads to precision loss
on Darwin. For example,
before: 2019-02-25 01:11:23.807627350 +0300 after: 2019-02-25 01:11:23.807627000 +0300 ^^^
This means we can't trick GHCi into thinking the file hasn't been changed by
restoring its old timestamp, as we cannot faithfully restore all digits.
The solution is to nullify the insignificant digits before the first :load
(cherry picked from commit f320f3b2f2f421af41b8f7588529ed46bbe6ccf3)