Disable logging to file during tests

Without this patch, salt-call emits warning when run as non-root:
[WARNING ] Failed to open log file, do you have permission to write to
/var/log/salt/minion

Change-Id: Ifde87fc95ca7565891c4cfc2ff1ef764ae9df9f2
diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index 6710a50..8c07e58 100755
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -18,7 +18,7 @@
 SALT_CONFIG_DIR=${SALT_CONFIG_DIR:-${BUILDDIR}/salt}
 SALT_CACHE_DIR=${SALT_CACHE_DIR:-${SALT_CONFIG_DIR}/cache}
 
-SALT_OPTS="${SALT_OPTS} --retcode-passthrough --local -c ${SALT_CONFIG_DIR}"
+SALT_OPTS="${SALT_OPTS} --retcode-passthrough --local -c ${SALT_CONFIG_DIR} --log-file=/dev/null"
 
 if [ "x${SALT_VERSION}" != "x" ]; then
     PIP_SALT_VERSION="==${SALT_VERSION}"