Improve the speed of do-nothing build.

As thockin found out here https://github.com/kubernetes/kubernetes/issues/24518,
vast majority of the do-nothing build time is spent in rebuilding the test
binaries. There is no staleness check support for test binaries.

This commit implements the staleness checks for test binaries and uses them
while building packages.

Tests are TBD. I am still trying to figure out how to test this.

hack/lib/golang.sh

@@ -114,6 +114,7 @@ kube::golang::test_targets() {
     cmd/linkcheck
     examples/k8petstore/web-server/src
     vendor/github.com/onsi/ginkgo/ginkgo
+    hack/cmd/teststale
     test/e2e/e2e.test
     test/e2e_node/e2e_node.test
   )
@@ -443,15 +444,25 @@ kube::golang::build_binaries_for_platform() {
     fi
   fi
 
+  teststale=$(kube::golang::output_filename_for_binary "hack/cmd/teststale" "${platform}")
   for test in "${tests[@]:+${tests[@]}}"; do
     local outfile=$(kube::golang::output_filename_for_binary "${test}" \
       "${platform}")
+
+    local testpkg="$(dirname ${test})"
+    if [[ "$(${teststale} -binary "${outfile}" -package "${testpkg}")" == "false" ]]; then
+      continue
+    fi
+    go install "${goflags[@]:+${goflags[@]}}" \
+        -ldflags "${goldflags}" \
+        "${testpkg}"
+
     mkdir -p "$(dirname ${outfile})"
     go test -c \
       "${goflags[@]:+${goflags[@]}}" \
       -ldflags "${goldflags}" \
       -o "${outfile}" \
-      "$(dirname ${test})"
+      "${testpkg}"
   done
 }