Makefile: Set CGO_ENABLED=0 for runtimetest
Makefile: Set CGO_ENABLED=0 for runtimetest

And also set '-installsuffix cgo'.  runtimetest is designed to be run
from inside a container, which may include a mount namespace that has
no linker or dynamic libraries.  These changes give us a static
runtimetest binary [1,2] which will work in those conditions.

[1]: https://golang.org/cmd/cgo/#hdr-Using_cgo_with_the_go_command
[2]: https://github.com/golang/go/issues/9344#issuecomment-69944514
     Subject: cmd/go: With Go 1.4, cannot statically link a program
       which imports net

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

liangchenye/ocitools,wking/ocitools-v2,opencontainers/runtime-tools,opencontainers/ocitools,opencontainers/ocitools,opencontainers/runtime-tools,liangchenye/ocitools,wking/ocitools-v2