Install test subpackages too

We weren't getting incremental builds because of new test only subpackages.  Our
voodoo combo of 'go install' and 'go test -c' didn't cache things like
'test/e2e_node/services'.  Add the '-i' flag to 'go test' to install test only
dependencies too.

hack/lib/golang.sh

@@ -547,7 +547,7 @@ kube::golang::build_binaries_for_platform() {
         "${testpkg}"
 
     mkdir -p "$(dirname ${outfile})"
-    go test -c \
+    go test -i -c \
       "${goflags[@]:+${goflags[@]}}" \
       -gcflags "${gogcflags}" \
       -ldflags "${goldflags}" \