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

tests/test-simplemerge.py

@@ -355,11 +355,5 b' bbb'
                          b'>>>>>>> THIS\r'.splitlines(True), list(m_lines))
 
 if __name__ == '__main__':
-    # hide the timer
-    import time
-    orig = time.time
-    try:
-        time.time = lambda: 0
-        unittest.main()
-    finally:
-        time.time = orig
+    import silenttestrunner
+    silenttestrunner.main(__name__)