Fix Process.WaitForExit premature timeout on Unix

The Process.WaitForExit implementation on Unix is mixing up units, resulting in WaitForExits with timeouts exiting much earlier than the specified timeout.  The code is using Stopwatch.GetTimestamp to get a current timestamp, subtracting the current timestamp from one gathered when the wait started, and then subtracting that difference from the specified timeout.  But the specified timeout is in milliseconds, and the timestamp is in ticks.  The fix just converts the difference from ticks to milliseconds before subtracing from the timeout value.

Author: stephentoub

src/System.Diagnostics.Process/src/System/Diagnostics/ProcessWaitState.Unix.cs

@@ -367,7 +367,7 @@ internal bool WaitForExit(int millisecondsTimeout)
             Debug.Assert(!Monitor.IsEntered(_gate));
 
             // Track the time the we start waiting.
-            long startTime = GetTimestamp();
+            long startTime = Stopwatch.GetTimestamp();
 
             // Polling loop
             while (true)
@@ -379,7 +379,7 @@ internal bool WaitForExit(int millisecondsTimeout)
                 // We're in a polling loop... determine how much time remains
                 int remainingTimeout = millisecondsTimeout == Timeout.Infinite ?
                     Timeout.Infinite :
-                    (int)Math.Max(millisecondsTimeout - (GetTimestamp() - startTime), 0);
+                    (int)Math.Max(millisecondsTimeout - ((Stopwatch.GetTimestamp() - startTime) / (double)Stopwatch.Frequency * 1000), 0);
 
                 lock (_gate)
                 {
@@ -511,11 +511,5 @@ internal bool WaitForExit(int millisecondsTimeout)
             });
         }
 
-        /// <summary>Gets a current time stamp.</summary>
-        private static long GetTimestamp()
-        {
-            return Stopwatch.GetTimestamp();
-        }
-
     }
 }