time: Rename udelay_test.c to test_udelay.c

Kees requested that this test module be renamed for consistency sake,
so this patch renames the udelay_test.c file (recently added to
tip/timers/core for 3.17) to test_udelay.c

Cc: Kees Cook <keescook@chromium.org>
Cc: Greg KH <greg@kroah.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linux-Next <linux-next@vger.kernel.org>
Cc: David Riley <davidriley@chromium.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>