fix: Use run start and end time to compute duration of test run (#29)

Fixes #26.

Author: codito

CHANGELOG.md

@@ -4,5 +4,6 @@
 
 * Refactor to support [core testlogger][]
 * Compatibility: minimum framework is netstandard1.5 and TestPlatform 15.5.0
+* Use test run start and end times for run duration reporting for assembly. See #26
 
 [core testlogger]: https://github.com/spekt/testlogger

src/Xunit.Xml.TestLogger/XunitXmlSerializer.cs

@@ -82,7 +82,7 @@ orderby resultsByType.Key
             int failed = 0;
             int skipped = 0;
             int errors = 0;
-            var time = TimeSpan.Zero;
+            var time = runConfiguration.EndTime - runConfiguration.StartTime;
 
             var element = new XElement("assembly");
             XElement errorsElement = new XElement("errors");
@@ -95,7 +95,6 @@ orderby resultsByType.Key
                 failed += collection.failed;
                 skipped += collection.skipped;
                 errors += collection.error;
-                time += collection.time;
 
                 element.Add(collection.element);
             }