added test for DisplayCommand

Author: Alexsey-VR

tests/DisplayCommandTest.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace Differ;
+
+use PHPUnit\Framework\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+use Differ\DisplayCommand;
+use Fixtures\CommandLineParsersStub;
+
+#[CoversClass(DisplayCommand::class)]
+class DisplayCommandTest extends TestCase
+{
+    public $displayCmd;
+    private $file1Content;
+    private $file2Content;
+
+    protected function setUp(): void
+    {
+        $this->displayCmd = new DisplayCommand();
+    
+        $this->file1Content =
+        [
+            "id" => "none",
+            "host" => "hexlet.io",
+            "timeout" => 50
+        ];
+        $this->file2Content =
+        [
+            "timeout" => 20,
+            "verbose" => 1,
+            "host" => "hexlet.io"
+        ];
+    }
+
+    public function testInstance()
+    {
+        $this->assertInstanceOf(DisplayCommand::class, $this->displayCmd);
+    }
+
+    public function testExecute()
+    {
+        $filesDiffCmd = $this->createConfiguredStub(
+            FilesDiffCommand::class,
+            [
+                'getFilesContent' =>
+                [
+                    "FILE1" => $this->file1Content,
+                    "FILE2" => $this->file2Content
+                ]
+            ]
+        );
+
+        $this->assertInstanceOf(DisplayCommand::class, $this->displayCmd
+                                                            ->execute($filesDiffCmd));
+    }
+
+    public function testGetDiffs()
+    {
+        $this->assertEquals("{\n}\n", $this->displayCmd->getDiffs());
+    }
+}