Add comprehensive testing infrastructure

This commit adds a complete testing setup for the prettier_action:

- Adds BATS (Bash Automated Testing System) testing framework
- Creates unit tests for _git_setup() and _git_changed() functions
- Creates plugin validation tests to ensure proper prettier plugin format
- Creates integration tests for end-to-end workflows
- Adds automated test runner script (tests/run_tests.sh)
- Adds GitHub Actions workflow for CI/CD testing
- Includes ShellCheck linting for bash scripts
- Updates README with comprehensive testing documentation
- Updates .gitignore to exclude test artifacts

Test coverage includes:
- Git configuration with different identity modes
- File change detection
- Plugin name validation (official, community, and scoped formats)
- Working directory handling
- node_modules cleanup
- package-lock.json restoration
- only_changed file filtering
- Dry run behavior

The test suite can be run locally with ./tests/run_tests.sh and runs
automatically on all pushes and pull requests.

tests/test_helper.bash

@@ -0,0 +1,78 @@
+#!/usr/bin/env bash
+
+# Test helper functions for prettier_action tests
+
+# Set up a temporary test directory
+setup_test_repo() {
+    export TEST_TEMP_DIR="$(mktemp -d)"
+    cd "$TEST_TEMP_DIR" || exit 1
+    git init
+    git config user.name "Test User"
+    git config user.email "test@example.com"
+}
+
+# Clean up temporary test directory
+teardown_test_repo() {
+    if [ -n "$TEST_TEMP_DIR" ] && [ -d "$TEST_TEMP_DIR" ]; then
+        rm -rf "$TEST_TEMP_DIR"
+    fi
+}
+
+# Create a sample file for testing
+create_sample_file() {
+    local filename="${1:-test.js}"
+    local content="${2:-const x=1;const y=2;}"
+    echo "$content" > "$filename"
+}
+
+# Create a sample package.json
+create_package_json() {
+    cat > package.json << 'EOF'
+{
+  "name": "test-project",
+  "version": "1.0.0",
+  "description": "Test project"
+}
+EOF
+}
+
+# Mock git environment variables for GitHub Actions
+mock_github_env() {
+    export GITHUB_ACTOR="${GITHUB_ACTOR:-test-actor}"
+    export GITHUB_ACTOR_ID="${GITHUB_ACTOR_ID:-12345}"
+    export GITHUB_ACTION_PATH="${GITHUB_ACTION_PATH:-/app}"
+    export GITHUB_BASE_REF="${GITHUB_BASE_REF:-main}"
+    export GITHUB_STEP_SUMMARY="${GITHUB_STEP_SUMMARY:-/dev/null}"
+    export INPUT_GITHUB_TOKEN="${INPUT_GITHUB_TOKEN:-test-token}"
+}
+
+# Set default input environment variables
+set_default_inputs() {
+    export INPUT_WORKING_DIRECTORY="${INPUT_WORKING_DIRECTORY:-}"
+    export INPUT_PRETTIER_VERSION="${INPUT_PRETTIER_VERSION:-latest}"
+    export INPUT_PRETTIER_OPTIONS="${INPUT_PRETTIER_OPTIONS:---write **/*.js}"
+    export INPUT_PRETTIER_PLUGINS="${INPUT_PRETTIER_PLUGINS:-}"
+    export INPUT_ALLOW_OTHER_PLUGINS="${INPUT_ALLOW_OTHER_PLUGINS:-false}"
+    export INPUT_CLEAN_NODE_FOLDER="${INPUT_CLEAN_NODE_FOLDER:-true}"
+    export INPUT_ONLY_CHANGED="${INPUT_ONLY_CHANGED:-false}"
+    export INPUT_ONLY_CHANGED_PR="${INPUT_ONLY_CHANGED_PR:-false}"
+    export INPUT_FILE_PATTERN="${INPUT_FILE_PATTERN:-*}"
+    export INPUT_DRY="${INPUT_DRY:-false}"
+    export INPUT_NO_COMMIT="${INPUT_NO_COMMIT:-false}"
+    export INPUT_SAME_COMMIT="${INPUT_SAME_COMMIT:-false}"
+    export INPUT_COMMIT_MESSAGE="${INPUT_COMMIT_MESSAGE:-Automated formatting}"
+    export INPUT_COMMIT_DESCRIPTION="${INPUT_COMMIT_DESCRIPTION:-}"
+    export INPUT_COMMIT_OPTIONS="${INPUT_COMMIT_OPTIONS:-}"
+    export INPUT_PUSH_OPTIONS="${INPUT_PUSH_OPTIONS:-}"
+    export INPUT_GIT_IDENTITY="${INPUT_GIT_IDENTITY:-actions}"
+}
+
+# Load a bash script without executing it (for testing functions)
+load_script_functions() {
+    local script_path="$1"
+    # Source only the function definitions, not the main program
+    # We extract functions by finding lines between function definitions and the main program block
+    sed -n '/^_git_setup/,/^}/p' "$script_path" > "$TEST_TEMP_DIR/functions.sh"
+    sed -n '/^_git_changed/,/^}/p' "$script_path" >> "$TEST_TEMP_DIR/functions.sh"
+    source "$TEST_TEMP_DIR/functions.sh"
+}