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/FileTest.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use Intervention\Image\File;
+use Intervention\Image\Tests\BaseTestCase;
+
+#[CoversClass(\Intervention\Image\File::class)]
+final class FileTest extends BaseTestCase
+{
+    public function testConstructor(): void
+    {
+        $file = new File('foo');
+        $this->assertInstanceOf(File::class, $file);
+    }
+
+    public function testSave(): void
+    {
+        $filename = __DIR__ . '/file_' . strval(hrtime(true)) . '.test';
+        $file = new File('foo');
+        $file->save($filename);
+        $this->assertTrue(file_exists($filename));
+        unlink($filename);
+    }
+
+    public function testToString(): void
+    {
+        $file = new File('foo');
+        $string = $file->toString();
+        $this->assertEquals('foo', $string);
+        $this->assertEquals('foo', (string) $string);
+    }
+
+    public function testToFilePointer(): void
+    {
+        $file = new File('foo');
+        $fp = $file->toFilePointer();
+        $this->assertIsResource($fp);
+    }
+
+    public function testSize(): void
+    {
+        $file = new File('foo');
+        $this->assertEquals(3, $file->size());
+    }
+}