make time measurements monotonic (#231)

- `time.time()` is the system time. "it's great" (tm)
  however it is not reliable for measuring time spent.
  it gives you the "current" time, but oh... time can move backwards
  (thanks NTP).

- `time.monotonic()` is a monotonic time source. "it's great" (tm)
  it only goes forward, making measurements a peach.
  however it cannot be used as a reference point.

xmlrunner/builder.py

@@ -58,7 +58,9 @@ def __init__(self, xml_doc, parent_context=None):
         """
         self.xml_doc = xml_doc
         self.parent = parent_context
-        self._start_time = self._stop_time = 0
+        self._start_time_m = 0
+        self._stop_time_m = 0
+        self._stop_time = 0
         self.counters = {}
 
     def element_tag(self):
@@ -72,12 +74,15 @@ def begin(self, tag, name):
         """
         self.element = self.xml_doc.createElement(tag)
         self.element.setAttribute('name', replace_nontext(name))
-        self._start_time = time.time()
+        self._start_time = time.monotonic()
 
     def end(self):
         """Closes this context (started with a call to `begin`) and creates an
         attribute for each counter and another for the elapsed time.
         """
+        # time.monotonic is reliable for measuring differences, not affected by NTP
+        self._stop_time_m = time.monotonic()
+        # time.time is used for reference point
         self._stop_time = time.time()
         self.element.setAttribute('time', self.elapsed_time())
         self.element.setAttribute('timestamp', self.timestamp())
@@ -120,7 +125,7 @@ def elapsed_time(self):
         """Returns the time the context took to run between the calls to
         `begin()` and `end()`, in seconds.
         """
-        return format(self._stop_time - self._start_time, '.3f')
+        return format(self._stop_time_m - self._start_time_m, '.3f')
 
     def timestamp(self):
         """Returns the time the context ended as ISO-8601-formatted timestamp.

xmlrunner/runner.py

@@ -62,9 +62,9 @@ def run(self, test):
             self.stream.writeln(result.separator2)
 
             # Execute tests
-            start_time = time.time()
+            start_time = time.monotonic()
             test(result)
-            stop_time = time.time()
+            stop_time = time.monotonic()
             time_taken = stop_time - start_time
 
             # Print results