test: repair stdlib/Dispatch on ASi

The conversion of the naonseconds to Mach ticks in multiples of Mach
timebase units alters the time.  This would fail on ASi where mach ticks
are not synonymous with nanoseconds.

Author: compnerd

test/stdlib/Dispatch.swift

@@ -278,7 +278,15 @@ DispatchAPI.test("DispatchTime.SchedulerTimeType.Stridable") {
 	    let time1 = DispatchQueue.SchedulerTimeType(.init(uptimeNanoseconds: 10000))
 	    let time2 = DispatchQueue.SchedulerTimeType(.init(uptimeNanoseconds: 10431))
 	    let addedTime = time2.distance(to: time1)
-	    expectEqual((10000 - 10431), addedTime.magnitude)
+            // The magnitude of the time sum is in nanosecond units.  Although
+            // the units match up, the internal representation of the
+            // DispatchTime may round up to multiples of Mach timebase.  This
+            // requires the difference being converted, which is performed here
+            // by constructing a `DispatchTime` from the delta.  However, the
+            // parameter is a `UInt64` which requires us to perform the negation
+            // manually.
+	    expectEqual(-Int(DispatchTime(uptimeNanoseconds: 10431 - 10000).uptimeNanoseconds),
+                        addedTime.magnitude)
 	}
 }