chore: Adjusting the testing scripts in .kokoro/samples.

.kokoro/samples/python3.6/common.cfg

@@ -10,7 +10,7 @@ action {
 # Specify which tests to run
 env_vars: {
     key: "RUN_TESTS_SESSION"
-    value: "py-3.6"
+    value: "samples-3.6"
 }
 
 # Declare build specific Cloud project.

.kokoro/samples/python3.7/common.cfg

@@ -10,7 +10,7 @@ action {
 # Specify which tests to run
 env_vars: {
     key: "RUN_TESTS_SESSION"
-    value: "py-3.7"
+    value: "samples-3.7"
 }
 
 # Declare build specific Cloud project.

.kokoro/samples/python3.8/common.cfg

@@ -10,7 +10,7 @@ action {
 # Specify which tests to run
 env_vars: {
     key: "RUN_TESTS_SESSION"
-    value: "py-3.8"
+    value: "samples-3.8"
 }
 
 # Declare build specific Cloud project.

.kokoro/test-samples-impl.sh

@@ -64,38 +64,32 @@ set +e
 # Use RTN to return a non-zero value if the test fails.
 RTN=0
 ROOT=$(pwd)
-# Find all requirements.txt in the samples directory (may break on whitespace).
-for file in samples/**/requirements.txt; do
-    cd "$ROOT"
-    # Navigate to the project folder.
-    file=$(dirname "$file")
-    cd "$file"
-
-    echo "------------------------------------------------------------"
-    echo "- testing $file"
-    echo "------------------------------------------------------------"
-
-    # Use nox to execute the tests for the project.
-    python3.6 -m nox -s "$RUN_TESTS_SESSION"
-    EXIT=$?
-
-    # If this is a periodic build, send the test log to the FlakyBot.
-    # See https://github.com/googleapis/repo-automation-bots/tree/master/packages/flakybot.
-    if [[ $KOKORO_BUILD_ARTIFACTS_SUBDIR = *"periodic"* ]]; then
-      chmod +x $KOKORO_GFILE_DIR/linux_amd64/flakybot
-      $KOKORO_GFILE_DIR/linux_amd64/flakybot
-    fi
-
-    if [[ $EXIT -ne 0 ]]; then
-      RTN=1
-      echo -e "\n Testing failed: Nox returned a non-zero exit code. \n"
-    else
-      echo -e "\n Testing completed.\n"
-    fi
-
-done
+
 cd "$ROOT"
 
+echo "------------------------------------------------------------"
+echo "- testing $file"
+echo "------------------------------------------------------------"
+
+# Use nox to execute the tests for the project.
+python3.6 -m nox -s "$RUN_TESTS_SESSION"
+EXIT=$?
+
+# If this is a periodic build, send the test log to the FlakyBot.
+# See https://github.com/googleapis/repo-automation-bots/tree/master/packages/flakybot.
+if [[ $KOKORO_BUILD_ARTIFACTS_SUBDIR = *"periodic"* ]]; then
+  chmod +x $KOKORO_GFILE_DIR/linux_amd64/flakybot
+  $KOKORO_GFILE_DIR/linux_amd64/flakybot
+fi
+
+if [[ $EXIT -ne 0 ]]; then
+  RTN=1
+  echo -e "\n Testing failed: Nox returned a non-zero exit code. \n"
+else
+  echo -e "\n Testing completed.\n"
+fi
+
+
 # Workaround for Kokoro permissions issue: delete secrets
 rm testing/{test-env.sh,client-secrets.json,service-account.json}