chore(ci): revamp parallel test outputs clarity and conciseness (#18002)

## Problem
The current output from the **package checking phase** (where we
determine if a package should be tested OR not) in parallel `system
tests` is noisy and difficult to read. It prints internal debug commands
and multiple lines of text for every package, making it hard to quickly
scan and see which packages are being tested or skipped.

## Solution
This Pull Request revamps the output format of the package checking
phase to be highly readable and aligned.
- Internal debug commands have been silenced.
- The output now uses a "State-First" format, where the decision (TEST
or SKIP) and the reason are displayed first, followed by the package
name and commit hash.
- The output is aligned in columns using `printf`, making vertical
scanning easy for humans and parsing easy for tools.

## Notes to Reviewers
- This change is purely cosmetic and does not alter the logic of package
selection or test execution.
- A harmless trigger comment was added to
`packages/google-cloud-testutils/setup.py` to force this package to be
tested in CI, allowing you to see the new output format in action. This
will be removed before merging.

## Example Output
```text
TEST [changed]     google-cloud-testutils c3bdf09
SKIP [no_changes]  google-cloud-tasks     c3bdf09
SKIP [no_changes]  google-cloud-storage   c3bdf09
```

## Pre-planning For Adhoc Testing:
While ad hoc testing is not yet merged to main and is not processed as a
part of this PR, the expectation is that a future PR will refine this
format when adhoc is added> At that time we will be able to determine
why a TEST state was chosen (changed, adhoc, or both if a package has
changes AND is inadvertantly included in the ad hoc list).

```text
TEST [changed]        google-cloud-testutils c3bdf09
TEST [adhoc]          google-cloud-tasks     c3bdf09
TEST [changed,adhoc]  google-cloud-tasks     c3bdf09
SKIP [no_changes]     google-cloud-storage   c3bdf09
```

Author: chalmerlowe

.kokoro/system.sh

@@ -263,16 +263,25 @@ for path in `find 'packages' \
     files_to_check=("${package_path}")
   fi
 
-  echo "checking changes with 'git diff ${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT} -- ${files_to_check[*]}'"
   set +e
-  # Passing the array expanded as arguments to git diff
+  # Passing the array expanded as arguments to git diff.
   package_modified=$(git diff "${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH}...${KOKORO_GITHUB_PULL_REQUEST_COMMIT}" -- "${files_to_check[@]}" | wc -l)
   set -e
 
-  if [[ "${package_modified}" -gt 0 || "$KOKORO_BUILD_ARTIFACTS_SUBDIR" == *"continuous"* ]]; then
+  states=()
+  [[ "${package_modified}" -gt 0 ]] && states+=("changed")
+  [[ "$KOKORO_BUILD_ARTIFACTS_SUBDIR" == *"continuous"* ]] && states+=("continuous")
+
+  # Join states with a comma
+  state_str=$(IFS=, ; echo "${states[*]}")
+
+  commit_hash="${KOKORO_GITHUB_PULL_REQUEST_COMMIT:-HEAD}"
+
+  if [[ ${#states[@]} -gt 0 ]]; then
+      printf "TEST %-20s %-40s %s\n" "[${state_str}]" "${package_name}" "${commit_hash}"
       PACKAGES_TO_TEST+=("$package_name")
   else
-      echo "No changes in ${package_name} and not a continuous build, skipping."
+      printf "SKIP %-20s %-40s %s\n" "[no_changes]" "${package_name}" "${commit_hash}"
   fi
 done