Added DomainUnloaded event.

When using the NUnit3 test adapter, the process doesn't exit, the app domain is simply unloaded. This means that the  ProcessExit event is never raised. The DomainUnload event is called always. To make sure we don't add double hits, we clear the _markers collection after writing (in case the event handler is triggered twice).

src/coverlet.core/CoverageTracker.cs

@@ -18,6 +18,7 @@ static CoverageTracker()
             _markers = new Dictionary<string, List<string>>();
             _markerFileCount = new Dictionary<string, int>();
             AppDomain.CurrentDomain.ProcessExit += new EventHandler(CurrentDomain_ProcessExit);
+            AppDomain.CurrentDomain.DomainUnload += new EventHandler(CurrentDomain_ProcessExit);
         }
 
         [ExcludeFromCoverage]
@@ -48,17 +49,22 @@ public static void MarkExecuted(string path, string marker)
         [ExcludeFromCoverage]
         public static void CurrentDomain_ProcessExit(object sender, EventArgs e)
         {
-            foreach (var kvp in _markers)
+            lock (_markers)
             {
-                using (var fs = new FileStream($"{kvp.Key}_compressed_{_markerFileCount[kvp.Key]}", FileMode.OpenOrCreate))
-                using (var gz = new GZipStream(fs, CompressionMode.Compress))
-                using (var sw = new StreamWriter(gz))
+                foreach (var kvp in _markers)
                 {
-                    foreach(var line in kvp.Value)
+                    using (var fs = new FileStream($"{kvp.Key}_compressed_{_markerFileCount[kvp.Key]}", FileMode.OpenOrCreate))
+                    using (var gz = new GZipStream(fs, CompressionMode.Compress))
+                    using (var sw = new StreamWriter(gz))
                     {
-                        sw.WriteLine(line);
+                        foreach(var line in kvp.Value)
+                        {
+                            sw.WriteLine(line);
+                        }
                     }
                 }
+
+                _markers.Clear();
             }
         }
     }