build: prevent system/sample-test from leaking credentials

Author: dpebot

handwritten/bigtable/.kokoro/samples-test.sh

@@ -26,7 +26,9 @@ cd $(dirname $0)/..
 
 # Run a pre-test hook, if a pre-samples-test.sh is in the project
 if [ -f .kokoro/pre-samples-test.sh ]; then
+    set +x
     . .kokoro/pre-samples-test.sh
+    set -x
 fi
 
 npm install

handwritten/bigtable/.kokoro/system-test.sh

@@ -26,7 +26,9 @@ cd $(dirname $0)/..
 
 # Run a pre-test hook, if a pre-system-test.sh is in the project
 if [ -f .kokoro/pre-system-test.sh ]; then
+    set +x
     . .kokoro/pre-system-test.sh
+    set -x
 fi
 
 npm install