Fix run_net_test.sh --norun.

This was likely broken by a change to make the script run
all_tests.sh if called with no arguments.

It is useful to keep this running because it is used by
parallel_tests.sh which is useful to find flaky tests.

Bug: 156144865
Test: treehugger
Test: build and run: ../net_test/run_net_test.sh
Test: build only: ../net_test/run_net_test.sh --norun
Test: run only: ../net_test/run_net_test.sh --nobuild bpf_test.py
Test: parallel tests: ../net_test/parallel_tests.sh 10 10 srcaddr_selection_test.py
Test: neither build nor run fails: ../net_test/run_net_test.sh --nobuild --norun
Original-Change: https://android-review.googlesource.com/1306567
Merged-In: I86162af99dea832d18206aae3dee1889b71c879d
Change-Id: I86162af99dea832d18206aae3dee1889b71c879d
diff --git a/net/test/run_net_test.sh b/net/test/run_net_test.sh
index ebfb15f..d1a66f5 100755
--- a/net/test/run_net_test.sh
+++ b/net/test/run_net_test.sh
@@ -178,16 +178,17 @@
 test_args=${@:2}
 
 function isRunningTest() {
-  [[ -n "$test" ]] && ! (( norun ))
+  ! (( norun ))
 }
 
 function isBuildOnly() {
-  [[ -z "$test" ]] && (( norun )) && ! (( nobuild ))
+  (( norun )) && ! (( nobuild ))
 }
 
 if ! isRunningTest && ! isBuildOnly; then
   echo "Usage:" >&2
-  echo "  $0 [--builder] [--readonly|--ro|--readwrite|--rw] [--nobuild] [--verbose] <test>" >&2
+  echo "  $0 [--builder] [--readonly|--ro|--readwrite|--rw] [--nobuild] [--verbose] [<test>]" >&2
+  echo "      - if [<test>] is not specified, run all_tests.sh" >&2
   echo "  $0 --norun" >&2
   exit 1
 fi