chore: run system tests only for modified packages (#14449)

Currently the system test runs for all packages. This PR adds logic to
check if there is a change to the package level `CHANGELOG.md` so that
we only runs system tests for packages that have changed. We are only
interested in `packages/<package>/CHANGELOG.md` changing as we only want
to run system tests on release PRs.

Fixes googleapis/librarian#2067

Author: parthea

.kokoro/system.sh

@@ -38,21 +38,35 @@ pwd
 # A file for running system tests
 system_test_script="${PROJECT_ROOT}/.kokoro/system-single.sh"
 
+# This is needed in order for `git diff` to succeed
+git config --global --add safe.directory $(realpath .)
+
 # Run system tests for each package with directory packages/*/tests/system
 for dir in `find 'packages' -type d -wholename 'packages/*/tests/system'`; do
   # Get the path to the package by removing the suffix /tests/system
   package=$(echo $dir | cut -f -2 -d '/')
-  echo "Running system tests for ${package}"
-  pushd ${package}
-  # Temporarily allow failure.
+
+  files_to_check=${package}/CHANGELOG.md
+
+  echo "checking changes with 'git diff "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT}" -- ${files_to_check}'"
   set +e
-  ${system_test_script}
-  ret=$?
+  package_modified=$(git diff "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT}" -- ${files_to_check} | wc -l)
   set -e
-  if [ ${ret} -ne 0 ]; then
-      RETVAL=${ret}
+  if [[ "${package_modified}" -eq 0 ]]; then
+      echo "no change detected in ${files_to_check}, skipping"
+  else
+      echo "change detected in ${files_to_check}"
+      echo "Running system tests for ${package}"
+      pushd ${package}
+      # Temporarily allow failure.
+      set +e
+      ${system_test_script}
+      ret=$?
+      set -e
+      if [ ${ret} -ne 0 ]; then
+          RETVAL=${ret}
+      fi
+      popd
   fi
-  popd
 done
-
 exit ${RETVAL}