runtime: mark TestTracebackArgs test functions nosplit

The argument liveness tests expect outputs where a dead stack slot
has a poisoned value. If the test function is preempted at the
prologue, it will go with the morestack code path which will spill
all the argument registers. Mark them nosplit to avoid that.

Should fix #49354.

Change-Id: I3b13e72e925748687a53c494bfaa70f07d9496fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/361211


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>