fix: checking existing argument with equal sign

tests/Unit/Plugins/Concerns/HandleArguments.php

@@ -0,0 +1,26 @@
+<?php
+
+use Pest\Plugins\Concerns\HandleArguments;
+
+test('method hasArgument', function (string $argument, bool $expectedResult) {
+    $obj = new class
+    {
+        use HandleArguments;
+    };
+
+    $arguments = [
+        '--long-argument',
+        'someValue',
+        '-a',
+        '--with-equal-sign=1337',
+    ];
+
+    expect($obj->hasArgument($argument, $arguments))->toBe($expectedResult);
+})->with([
+    ['--long-argument', true],
+    ['-a', true],
+    ['--with-equal-sign', true],
+    ['someValue', true],
+    ['--a', false],
+    ['--undefined-argument', false],
+]);