KT-58046 Use saturated math in LongTimeMark implementation

- keep offset in range (-1..+1) of time source units
- when adding big or infinite offset, saturate startedAt instead
- displace initial reading to zero, similar to MonotonicTimeSource
- fix the zero reading in TestTimeSource by calling markNow in constructor
- use more precise reading adjustment in TestTimeSource for big durations
- add a note about comparable time marks for AbstractLongTimeSource and TestTimeSource

libraries/stdlib/test/time/TestTimeSourceTest.kt

@@ -41,8 +41,10 @@ class TestTimeSourceTest {
         run {
             val timeSource = TestTimeSource()
             timeSource += moderatePositiveDuration
+            val mark = timeSource.markNow()
             // does not overflow even if duration doesn't fit in long, but the result fits
             timeSource += -moderatePositiveDuration - Long.MAX_VALUE.nanoseconds
+            assertEquals(-(moderatePositiveDuration + Long.MAX_VALUE.nanoseconds), mark.elapsedNow())
         }
     }