make test: fix support for PARALLEL

There was an env variable PARALLEL and a -p command line flag,
but the value then wasn't passed on to "go test".

The new default is to not set any explicit parallelism, which
matches the prior (accidental?) behavior of ignoring PARALLEL.

hack/make-rules/test.sh

@@ -101,7 +101,7 @@ isnum() {
   [[ "$1" =~ ^[0-9]+$ ]]
 }
 
-PARALLEL="${PARALLEL:-1}"
+PARALLEL="${PARALLEL:--1}"
 while getopts "hp:i:" opt ; do
   case ${opt} in
     h)
@@ -175,6 +175,10 @@ if [[ -n "${KUBE_RACE}" ]] ; then
   goflags+=("${KUBE_RACE}")
 fi
 
+if [[ "${PARALLEL}" -gt 0 ]]; then
+  goflags+=(-p "${PARALLEL}")
+fi
+
 junitFilenamePrefix() {
   if [[ -z "${KUBE_JUNIT_REPORT_DIR}" ]]; then
     echo ""