Refactor EntityFile "unit" test fails

Follow-up to #2695297: Refactor EntityFile and use process plugins instead

Problem/Motivation

Unit test suite fails out of the box using the phpunit runner.

There were 4 errors:

1) Drupal\Tests\migrate\Unit\process\CopyFileTest::testSuccessfulCopies
mkdir(): No such file or directory

/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:67
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:278
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:232
/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:39
/var/www/testing/core/modules/migrate/tests/src/Unit/process/CopyFileTest.php:34

2) Drupal\Tests\migrate\Unit\process\CopyFileTest::testSuccessfulMoves
mkdir(): No such file or directory

/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:67
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:278
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:232
/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:39
/var/www/testing/core/modules/migrate/tests/src/Unit/process/CopyFileTest.php:34

3) Drupal\Tests\migrate\Unit\process\CopyFileTest::testNonExistentSourceFile
mkdir(): No such file or directory

/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:67
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:278
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:232
/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:39
/var/www/testing/core/modules/migrate/tests/src/Unit/process/CopyFileTest.php:34

4) Drupal\Tests\migrate\Unit\process\CopyFileTest::testRenameFile
mkdir(): No such file or directory

/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:67
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:278
/var/www/testing/core/tests/Drupal/KernelTests/KernelTestBase.php:232
/var/www/testing/core/tests/Drupal/KernelTests/Core/File/FileTestBase.php:39
/var/www/testing/core/modules/migrate/tests/src/Unit/process/CopyFileTest.php:34

The listed failing tests are actually kernel tests so in the wrong namespace and so run as part of the wrong test suite. There may be an issue with kernel tests requiring a directory that doesn't exist as well.

Remaining tasks

Write patch to move new migate unit tests into kernel namespace.

User interface changes

n/a

API changes

n/a

Data model changes

n/a