Loosen the variation type disabling on the product type form

It is a feature, because changing this after creating products would cause breakage.
However, the check is too strict. We can allow changing the variation type as long as you don't have any product entities created.
That would satisfy the clone use case.

This is config, it's all serialized, there's no db column to change.

The fix looks good. Tests need a bit of cleanup:

+    $this->assertFalse($this->getSession()->getPage()->findField('variationType')->hasAttribute('disabled'));

This line is too long and hard to read. Let's assign $variation_type_field on the line above, then use that in the assert:

  $variation_type_field = $this->getSession()->getPage()->findField('variationType');
  $this->assertFalse($variation_type_field->hasAttribute('disabled'));

+    // Cannot change the variation of a product type once a product has been
+    // created.
+    $values = [
+      'type' => 'default',
+      'title' => 'Test product',
+      'sku' => 'testsku',
+    ];
+    \Drupal::entityTypeManager()->getStorage('commerce_product')->create($values)->save();

Should use $this->createEntity() for consistency.

EDIT:

+    $product = $this->createEntity('commerce_product', [
+      'type' => 'default',
+      'title' => 'Test product',
+      'sku' => 'testsku',
+    ]);

Missed the SKU field, no such thing on a product. Can be fixed on commit.

Done. Thanks, Andras!