Tests for Timer.sleep().

test/timer/sleep_fibers.wren

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+import "timer" for Timer
+
+Fiber.new {
+  Timer.sleep(2)
+  IO.print("a")
+}.call()
+
+Fiber.new {
+  Timer.sleep(1)
+  IO.print("b")
+}.call()
+
+IO.print("main")
+Timer.sleep(3)
+IO.print("done")
+// expect: main
+// expect: b
+// expect: a
+// expect: done