Squelch output for a failure case in mixed_mode_test

Currently, the expected output shows as the main text for
unrelated presubmit failures.
Bug: 278748024
Test: build/soong/tests/mixed_mode_test.sh

Change-Id: I553a82597c7c7791a822e330eb9b2663f35567a1

tests/mixed_mode_test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash -eu
+#!/bin/bash
 
 set -o pipefail
 
@@ -88,12 +88,12 @@ EOF
     fail "Bazel actions not found for force-enabled module"
   fi
 
-  local exit_code=`run_soong --bazel-force-enabled-modules=unenabled-touch-file nothing`
+  unused=`run_soong --bazel-force-enabled-modules=unenabled-touch-file nothing >/dev/null`
 
-  if [[ $exit_code -ne 1 ]]; then
+  if [[ $? -ne 1 ]]; then
     fail "Expected failure due to force-enabling an unenabled module "
   fi
 }
 
 
-scan_and_run_tests
+scan_and_run_tests