Variant sort isn't consistent in PHP 5 and PHP 7

When running ctools test suite with php7 we get the following failure.

There was 1 failure:

1) Drupal\Tests\ctools\Unit\VariantCollectionTraitTest::testGetVariantsSort
Failed asserting that Array &0 (
    'foo' => 'foo'
    'bar' => 'bar'
) is identical to Array &0 (
    'bar' => 'bar'
    'foo' => 'foo'
).

Since the plugins created for the variant tests here don't have a weight, they get the "identical" sort behavior. This changed in PHP 7 to not swap the values though[ref]. I think the likely solution is to fall back on sorting by id when there isn't a weight so variants sort consistently in all versions of PHP and identical weighted variants have a logic order.