Add ClockMark and measureTime tests with test clock

libraries/stdlib/test/time/ClockMarkTest.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2010-2019 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.
+ */
+
+package test.time
+
+import kotlin.test.*
+import kotlin.time.*
+
+class ClockMarkTest {
+
+    @Test
+    fun adjustment() {
+        val clock = TestClock(unit = DurationUnit.NANOSECONDS)
+
+        val mark = clock.mark()
+        val markFuture1 = mark + 1.milliseconds
+        val markFuture2 = mark - (-1).milliseconds
+
+        val markPast1 = mark - 1.milliseconds
+        val markPast2 = markFuture1 + (-2).milliseconds
+
+        clock.reading = 500_000L
+
+        val elapsed = mark.elapsed()
+        val elapsedFromFuture = elapsed - 1.milliseconds
+        val elapsedFromPast = elapsed + 1.milliseconds
+
+        assertEquals(0.5.milliseconds, elapsed)
+        assertEquals(elapsedFromFuture, markFuture1.elapsed())
+        assertEquals(elapsedFromFuture, markFuture2.elapsed())
+
+        assertEquals(elapsedFromPast, markPast1.elapsed())
+        assertEquals(elapsedFromPast, markPast2.elapsed())
+    }
+}