[Test] Fix unstable Backtracing unit test

There was a problem in CrashWithThreads where it failed about 10% of the time. This fixes it.
rdar://165728982

test/Backtracing/CrashWithThreads.swift

@@ -57,7 +57,7 @@ func spawnThread(_ shouldCrash: Bool) {
 
 let crashingThreadIndex = (1..<10).randomElement()
 
-for threadIndex in 0..<10 {
+for threadIndex in 1..<10 {
   spawnThread(threadIndex == crashingThreadIndex)
 }