Introduce DUTB helper methods for working with rendered content + FieldPluginUnitTestBase

In short:

Move the helper methods from the test in #1152216-16: (Long) text field (textarea) with "Default" formatter does not retain newlines into DrupalUnitTestBase + a new FieldPluginUnitTestBase.

#1932382: Use DrupalUnitTestBase for Field API tests. introduced some additional + similar helper methods in a FieldUnitTestBase (DUTB).