[BUGFIX] Prevent usage of non existing php function in PHP 7.2 tests

The function in question is array_key_first, which was introduced with
PHP 7.3.0. It is not available for PHP 7.2 testing, so a workaround
was introduced.

Resolves: #92343
Releases: master, 10.4
Change-Id: Id0eaceefb4fb5581b8f3a25de8d43819b9389d76
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65768


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/dashboard/Tests/Unit/DashboardPresetRegistryTest.php

@@ -47,7 +47,7 @@ class DashboardPresetRegistryTest extends UnitTestCase
         $presets = $this->subject->getDashboardPresets();
         self::assertCount(1, $presets);
 
-        self::assertEquals('dashboardPreset-fallback', array_key_first($presets));
+        self::assertSame('dashboardPreset-fallback', key($presets));
         self::assertInstanceOf(DashboardPreset::class, reset($presets));
     }