PHPUnit 10 Migration (#1302)

* Bump PHPUnit dependencies

* Set return type of base TestCase methods

From the [PHPUnit 8 release notes][1], the `TestCase` methods below now declare a `void` return type:

- `setUpBeforeClass()`
- `setUp()`
- `assertPreConditions()`
- `assertPostConditions()`
- `tearDown()`
- `tearDownAfterClass()`
- `onNotSuccessfulTest()`

[1]: https://phpunit.de/announcements/phpunit-8.html

* Ignore PHPUnit cache folder

* Adopt PHP attributes in test classes

* Declare data providers as `static`

* Add return types to test methods

* Define test classes as `final`

* Migrate phpunit.xml to phpunit 10

* Correct phpunit attribute class name

* Rename base test class

* Restructure test folders

* Fix test image paths

* Only set rules for php files in .editorconfig

* Remove php unit flag in local test env

---------

Co-authored-by: Shift <shift@laravelshift.com>

tests/Unit/Geometry/PixelTest.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Unit\Geometry;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use Intervention\Image\Geometry\Pixel;
+use Intervention\Image\Interfaces\ColorInterface;
+use Intervention\Image\Tests\BaseTestCase;
+use Mockery;
+
+#[CoversClass(\Intervention\Image\Geometry\Pixel::class)]
+final class PixelTest extends BaseTestCase
+{
+    public function testSetGetBackground(): void
+    {
+        $color = Mockery::mock(ColorInterface::class);
+        $pixel = new Pixel($color, 10, 12);
+        $result = $pixel->setBackgroundColor($color);
+        $this->assertInstanceOf(ColorInterface::class, $pixel->backgroundColor());
+        $this->assertInstanceOf(Pixel::class, $result);
+    }
+}