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/MeasureTimeTest.kt

@@ -47,11 +47,12 @@ class MeasureTimeTest {
     }
 
 
-    @Test fun measureTimeTestClock() {
-        val clock = TestClock(unit = DurationUnit.NANOSECONDS)
+    @Test
+    fun measureTimeTestClock() {
+        val clock = TestClock()
         val expectedNs = Random.nextLong(1_000_000_000L)
         val elapsed = clock.measureTime {
-            clock.reading += expectedNs
+            clock += expectedNs.nanoseconds
         }
 
         assertEquals(expectedNs.nanoseconds, elapsed)
@@ -59,7 +60,7 @@ class MeasureTimeTest {
         val expectedResult: Long
 
         val (result, elapsed2) = clock.measureTimedValue {
-            clock.reading += expectedNs
+            clock += expectedNs.nanoseconds
             expectedResult = expectedNs
             expectedNs
         }