Advance TestTimeSource more precisely

Change durations in its test so that they hold precise number
of nanoseconds

libraries/stdlib/src/kotlin/time/TimeSources.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -84,14 +84,14 @@ public class TestTimeSource : AbstractLongTimeSource(unit = DurationUnit.NANOSEC
      * @throws IllegalStateException when the reading value overflows as the result of this operation.
      */
     public operator fun plusAssign(duration: Duration) {
-        val delta = duration.toDouble(unit)
-        val longDelta = delta.toLong()
+        val longDelta = duration.toLong(unit)
         reading = if (longDelta != Long.MIN_VALUE && longDelta != Long.MAX_VALUE) {
             // when delta fits in long, add it as long
             val newReading = reading + longDelta
             if (reading xor longDelta >= 0 && reading xor newReading < 0) overflow(duration)
             newReading
         } else {
+            val delta = duration.toDouble(unit)
             // when delta is greater than long, add it as double
             val newReading = reading + delta
             if (newReading > Long.MAX_VALUE || newReading < Long.MIN_VALUE) overflow(duration)