os: embed "sleep 1" within the test binary itself.

This is an alternative to http://golang.org/cl/4150,
and is motivated by a review comment on that CL.

testKillProcess() tries to build and run the Go equivalent
for "sleep 1". This doesn't work for testing cross compilers
since the Go compiler is not available on the targets. This
change embeds the "sleep 1" functionality within the "os.test"
binary itself.

Change-Id: I6bad513deaa6c9e2704e70319098eb4983f1bb23
Reviewed-on: https://go-review.googlesource.com/4190


Reviewed-by: Ian Lance Taylor <iant@golang.org>