Issue #1872540 by xjm: Provide a test helper method for creating block instances.

...r/lib/Drupal/aggregator/Tests/AggregatorRenderingTest.php

...aggregator/lib/Drupal/aggregator/Tests/ImportOpmlTest.php

core/modules/block/lib/Drupal/block/Tests/BlockCacheTest.php

...es/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php

.../modules/block/lib/Drupal/block/Tests/BlockHtmlIdTest.php

...s/block/lib/Drupal/block/Tests/BlockInvalidRegionTest.php

...lock/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php

core/modules/book/lib/Drupal/book/Tests/BookTest.php

...les/comment/lib/Drupal/comment/Tests/CommentBlockTest.php

core/modules/forum/lib/Drupal/forum/Tests/ForumBlockTest.php

...ules/forum/lib/Drupal/forum/Tests/ForumNodeAccessTest.php

...guage/lib/Drupal/language/Tests/LanguageSwitchingTest.php

...upal/language/Tests/LanguageUILanguageNegotiationTest.php

core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php

...es/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php

...les/node/lib/Drupal/node/Tests/NodeSyndicateBlockTest.php

...modules/openid/lib/Drupal/openid/Tests/OpenIDTestBase.php

core/modules/poll/lib/Drupal/poll/Tests/PollBlockTest.php

...odules/search/lib/Drupal/search/Tests/SearchBlockTest.php

.../lib/Drupal/search/Tests/SearchConfigSettingsFormTest.php