Bug 574180 - Improve
org.eclipse.test.TracingSuite.DumpTask.dumpStackTraces()

Use similar (better) format from
org.eclipse.jdt.core.tests.model.FreezeMonitor.expectCompletionIn(long)

Change-Id: Ia397d0a52f36ed370c43a5959ca931e0bad52b10
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.releng/+/181909
Tested-by: Platform Bot <platform-bot@eclipse.org>