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

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

tests/Unit/Drivers/Gd/Decoders/AbstractDecoderTest.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Unit\Drivers\Gd\Decoders;
+
+use Intervention\Image\Drivers\Gd\Decoders\AbstractDecoder;
+use Intervention\Image\Tests\BaseTestCase;
+use Mockery;
+
+final class AbstractDecoderTest extends BaseTestCase
+{
+    public function testGetMediaTypeFromFilePath(): void
+    {
+        $decoder = Mockery::mock(AbstractDecoder::class)->makePartial();
+        $this->assertEquals('image/jpeg', $decoder->getMediaTypeByFilePath($this->getTestImagePath('test.jpg')));
+    }
+
+    public function testGetMediaTypeFromFileBinary(): void
+    {
+        $decoder = Mockery::mock(AbstractDecoder::class)->makePartial();
+        $this->assertEquals('image/jpeg', $decoder->getMediaTypeByBinary($this->getTestImageData('test.jpg')));
+    }
+}