[BUGFIX] Ensure to take test instance as class mode instance

The bugfix backport to ensure correct system environment path
building to fix issue TYPO3#577 missed to correctly set the flag
for non-composer mode for TYPO3 v12 and lead to wrong path
calculations and followup issues within functional tests.

This is fixed by handing over a correct override value within
the functional test bootstrap.

[1] TYPO3#577
[2] TYPO3#633

Resolves: TYPO3#658
Related: TYPO3#633
Related: TYPO3@409c2b8
Releases: 8

Backported from dd7505b

Classes/Core/SystemEnvironmentBuilder.php

@@ -40,13 +40,15 @@
  */
 class SystemEnvironmentBuilder extends CoreSystemEnvironmentBuilder
 {
-    public static function run(int $entryPointLevel = 0, int $requestType = CoreSystemEnvironmentBuilder::REQUESTTYPE_FE, bool $composerMode = false)
+    protected static ?bool $composerMode = null;
+    public static function run(int $entryPointLevel = 0, int $requestType = CoreSystemEnvironmentBuilder::REQUESTTYPE_FE, ?bool $composerMode = null)
     {
+        static::$composerMode = $composerMode;
         CoreSystemEnvironmentBuilder::run($entryPointLevel, $requestType);
         Environment::initialize(
             Environment::getContext(),
             Environment::isCli(),
-            $composerMode,
+            static::usesComposerClassLoading(),
             Environment::getProjectPath(),
             Environment::getPublicPath(),
             Environment::getVarPath(),
@@ -55,4 +57,9 @@ public static function run(int $entryPointLevel = 0, int $requestType = CoreSyst
             Environment::isWindows() ? 'WINDOWS' : 'UNIX'
         );
     }
+
+    protected static function usesComposerClassLoading(): bool
+    {
+        return static::$composerMode ?? parent::usesComposerClassLoading();
+    }
 }

Classes/Core/Testbase.php

@@ -682,7 +682,7 @@ public function setUpBasicTypo3Bootstrap($instancePath): ContainerInterface
         GeneralUtility::purgeInstances();
 
         $classLoader = require $this->getPackagesPath() . '/autoload.php';
-        SystemEnvironmentBuilder::run(1, SystemEnvironmentBuilder::REQUESTTYPE_BE | SystemEnvironmentBuilder::REQUESTTYPE_CLI);
+        SystemEnvironmentBuilder::run(1, SystemEnvironmentBuilder::REQUESTTYPE_BE | SystemEnvironmentBuilder::REQUESTTYPE_CLI, false);
         $container = Bootstrap::init($classLoader);
         // Make sure output is not buffered, so command-line output can take place and
         // phpunit does not whine about changed output bufferings in tests.