Add `GlobalCommandTest.php` (#12073)

* Add `GlobalCommandTest.php`

- `testGlobal` to check `COMPOSER_HOME` is followed correctly + check
  `COMPOSER` is unset.
- `testNotCreateHome` to test handling invalid `COMPOSER_HOME`.

* Add error string for non obvious test case

* Clean up env vars and minor code style changes

---------

Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>

tests/Composer/Test/Command/GlobalCommandTest.php

@@ -0,0 +1,80 @@
+<?php declare(strict_types=1);
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Test\Command;
+
+use Composer\Test\TestCase;
+use Composer\Util\Platform;
+
+class GlobalCommandTest extends TestCase
+{
+    public function tearDown(): void
+    {
+        parent::tearDown();
+        Platform::clearEnv('COMPOSER_HOME');
+        Platform::clearEnv('COMPOSER');
+    }
+    
+    public function testGlobal(): void
+    {
+        $script = '@php -r \'echo getenv("COMPOSER") . PHP_EOL;\'';
+        $fakeComposer = 'TMP_COMPOSER.JSON';
+        $composerHome = $this->initTempComposer(
+            [
+                "scripts" => [
+                    "test-script" => $script,
+                ],
+            ]
+        );
+
+        Platform::putEnv('COMPOSER_HOME', $composerHome);
+        Platform::putEnv('COMPOSER', $fakeComposer);
+
+        $dir = self::getUniqueTmpDirectory();
+        chdir($dir);
+
+        $appTester = $this->getApplicationTester();
+        $appTester->run([
+            'command' => 'global',
+            'command-name' => 'test-script',
+            '--no-interaction' => true,
+        ]);
+
+        $display = $appTester->getDisplay(true);
+
+        self::assertStringContainsString(
+            'Changed current directory to ' . $composerHome,
+            $display
+        );
+        self::assertStringContainsString($script, $display);
+        self::assertStringNotContainsString($fakeComposer, $display, '$COMPOSER is not unset by global command');
+    }
+
+    public function testCannotCreateHome(): void
+    {
+        $dir = self::getUniqueTmpDirectory();
+        $filename = $dir . '/file';
+        file_put_contents($filename, '');
+
+        Platform::putEnv('COMPOSER_HOME', $filename);
+
+        self::expectException(\RuntimeException::class);
+        $this->expectExceptionMessage($filename . ' exists and is not a directory.');
+
+        $appTester = $this->getApplicationTester();
+        $appTester->run([
+            'command' => 'global',
+            'command-name' => 'test-script',
+            '--no-interaction' => true,
+        ]);
+    }
+}