test: rename live_syscall.go to live_uintptrkeepalive.go

test: rename live_syscall.go to live_uintptrkeepalive.go

CL 388095 will change this file significantly. Move it preemptively to
ensure git tracks the move properly.

For #51087

Change-Id: I1408aecf8675723041b64e54cf44cdec38cc655c
Reviewed-on: https://go-review.googlesource.com/c/go/+/388094
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>