Add tests for all the formatters

Love that this is a unit-test, so will be quick. Brilliant work!
Just one question:

+++ b/src/Tests/DynamicEntityReferenceFormatterTest.php
@@ -0,0 +1,319 @@
+    $expected_rendered_name_field_1 = '<div class="field field-entity-test--name field-name-name field-type-string field-label-hidden">
+    <div class="field-items">
+          <div class="field-item">' . $this->referencedEntity->label() . '</div>
+      </div>
+</div>
+';
+    $expected_rendered_body_field_1 = '<div class="clearfix field field-entity-test--body field-name-body field-type-text field-label-above">
+      <div class="field-label">Body</div>
+    <div class="field-items">
+          <div class="field-item"><p>Hello, world!</p></div>
+      </div>
+</div>

Does this make the test fragile if further css changes in core remove some of these divs?

ok