Issue #2683579 by alexpott, benjy: Improve migrate UI tests

...es/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php

...es/migrate_drupal_ui/src/Tests/d6/MigrateUpgrade6Test.php

...c/Tests/d6/files/core/modules/simpletest/files/html-1.txt

.../Tests/d6/files/core/modules/simpletest/files/image-1.png

.../Tests/d6/files/core/modules/simpletest/files/image-2.jpg

...sts/d6/files/core/modules/simpletest/files/image-test.gif

...sts/d6/files/core/modules/simpletest/files/image-test.jpg

...sts/d6/files/core/modules/simpletest/files/image-test.png

...grate_drupal_ui/src/Tests/d6/files/tmp/some-temp-file.jpg

...es/migrate_drupal_ui/src/Tests/d7/MigrateUpgrade7Test.php

...rupal_ui/src/Tests/d7/files/sites/default/files/cube.jpeg