Remove unused local variable and simpletest_generate_file() function call from core/modules/file/tests/src/Functional/FileFieldDisplayTest.php

Problem/Motivation

While looking at \Drupal\Tests\file\Functional\FileFieldDisplayTest::testNodeDisplay() I noticed the following snippet:

    $test_file = $this->getTestFile('text');
    simpletest_generate_file('escaped-&-text', 64, 10, 'text');
    $test_file = File::create([
      'uri' => 'public://escaped-&-text.txt',
      'name' => 'escaped-&-text',
      'filesize' => filesize('public://escaped-&-text.txt'),
    ]);

As you can see that $test_file variable is being overwritten by File::create() call and there is a simpletest_generate_file() call which should be avoided because this is a PHPUnit test.

Proposed resolution

1. Remove $test_file = $this->getTestFile('text');
2. replace simpletest_generate_file() call with its equivalent $this->generateFile()

Remaining tasks

Write patch

User interface changes

N/A

API changes

N/A

Data model changes

N/A