Introduce TestClock.plusAssign(Duration) and hide implementation details

TestClock.plusAssign(Duration) is for advancing TestClock time.
Hide reading value, do not allow to provide initial reading,
fix clock unit to nanoseconds at construction time.

libraries/stdlib/test/time/ClockMarkTest.kt

@@ -12,7 +12,7 @@ class ClockMarkTest {
 
     @Test
     fun adjustment() {
-        val clock = TestClock(unit = DurationUnit.NANOSECONDS)
+        val clock = TestClock()
 
         val mark = clock.mark()
         val markFuture1 = mark + 1.milliseconds
@@ -21,7 +21,7 @@ class ClockMarkTest {
         val markPast1 = mark - 1.milliseconds
         val markPast2 = markFuture1 + (-2).milliseconds
 
-        clock.reading = 500_000L
+        clock += 500_000.nanoseconds
 
         val elapsed = mark.elapsed()
         val elapsedFromFuture = elapsed - 1.milliseconds