tests: use silenttestrunner in test-simplemerge.py
The time monkeypatching doesn't appear to work reliably in
Python 3, possibly due to unittest using a different time
function. This makes the test intermittent due to the
execution time not always being `0.00s`.
We have our own wrapper around unittest for more deterministic
test output. So let's use it.
As a bonus, all test output disappeared, so we can remove the
.out file!
Differential Revision: https://phab.mercurial-scm.org/D6921