fix: Ensure the debug flag is not forwarded

If the PHPUnit test is executed via a Makefile command with the `--debug` flag, for example for this library:

```
make phpunit --debug
```

Then the `MAKEFLAGS` environment variable is updated and forwarded to
the processes in which the test make commands are executed. This result
in a different output which can easily break tests.

.github/workflows/tests.yaml

@@ -36,6 +36,31 @@ jobs:
  - name: "Run tests"
  run: "make phpunit"
 
+ debug-test:
+ runs-on: "ubuntu-latest"
+ name: "Tests with PHP ${{ matrix.php }} and the make `--debug` flag"
+ strategy:
+ fail-fast: true
+ matrix:
+ php:
+ - "8.1"
+
+ steps:
+ - name: "Check out repository code"
+ uses: "actions/checkout@v4"
+
+ - name: "Setup PHP"
+ uses: "shivammathur/setup-php@v2"
+ with:
+ php-version: "${{ matrix.php }}"
+ tools: "composer"
+
+ - name: "Install Composer dependencies"
+ uses: "ramsey/composer-install@v2"
+
+ - name: "Run tests"
+ run: "make phpunit --debug"
+
  infection:
  runs-on: "ubuntu-latest"
  name: "Infection with PHP ${{ matrix.php }}"
@@ -70,6 +95,7 @@ jobs:
  runs-on: ubuntu-latest
  needs:
  - tests
+ - debug-test
  - infection
  if: always()
  steps:

src/Test/BaseMakefileTestCase.php

@@ -40,9 +40,13 @@
 use Fidry\Makefile\Rule;
 use PHPUnit\Framework\TestCase;
 use Safe\Exceptions\ExecException;
+use function array_filter;
 use function dirname;
 use function error_clear_last;
+use function explode;
 use function function_exists;
+use function getenv;
+use function implode;
 use function Safe\chdir;
 use function Safe\file_get_contents;
 use function Safe\getcwd;
@@ -129,6 +133,12 @@ final protected static function executeMakeCommand(string $commandName): string
  // TODO: remove this as we remove support for PHP 7.4 and Safe v1
  final protected static function executeCommand(string $command): string
  {
+ $command = sprintf(
+ 'MAKEFLAGS=%s %s',
+ self::getNonDebugMakeFlags(),
+ $command,
+ );
+
  if (function_exists('Safe\shell_exec')) {
  return shell_exec($command);
  }
@@ -144,6 +154,18 @@ final protected static function executeCommand(string $command): string
  return $safeResult;
  }
 
+ final protected static function getNonDebugMakeFlags(): string
+ {
+ $makeFlags = (string) getenv('MAKEFLAGS');
+
+ $nonDebugFlags = array_filter(
+ explode(' ', $makeFlags),
+ static fn (string $flag) => str_starts_with('--debug=', $flag),
+ );
+
+ return implode(' ', $nonDebugFlags);
+ }
+
  private static function getTimeout(): string
  {
  try {