Issue #2631698 followup by heddn, neclimdul: Fix sub-optimal DX in MigrateFieldInterface

core/modules/datetime/tests/src/Unit/Plugin/migrate/field/d6/DateFieldTest.php

@@ -29,6 +29,7 @@ protected function setUp() {
   /**
    * Tests an Exception is thrown when the field type is not a known date type.
    *
+   * @runInSeparateProcess
    * @expectedDeprecation DateField is deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.x. Use \Drupal\datetime\Plugin\migrate\field\DateField instead.
    */
   public function testUnknownDateType($method = 'defineValueProcessPipeline') {

core/modules/file/tests/src/Unit/Plugin/migrate/field/d7/ImageFieldTest.php

@@ -44,6 +44,7 @@ protected function setUp() {
   }
 
   /**
+   * @runInSeparateProcess
    * @covers ::defineValueProcessPipeline
    * @expectedDeprecation ImageField is deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use \Drupal\image\Plugin\migrate\field\d7\ImageField instead. See https://www.drupal.org/node/2936061.
    */