Use GitLab CI and fix tests

I don't know the module or pharborist at all, but I'm expecting to have a D7 upgrade project soon and thought I'd try and help push things forward a little.

The test error seems to be legit, I get it locally.

1) Drupal\Tests\drupalmoduleupgrader\Unit\Plugin\DMU\Fixer\DeleteTest::test
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
 '<?php\n
 \n
-'
+function foo_permission() {\n
+  return array(\n
+    'bazify' => array(\n
+      'title' => 'Do snazzy bazzy things',\n
+    ),\n
+  );\n
+}'
/builds/issue/drupalmoduleupgrader-3465500/vendor/phpunit/phpunit/src/Framework/Constraint/Equality/IsEqual.php:95
/builds/issue/drupalmoduleupgrader-3465500/tests/src/Unit/Plugin/DMU/Fixer/DeleteTest.php:50

As far as I can tell it's down to \Drupal\drupalmoduleupgrader\Plugin\DMU\Fixer\Delete::execute() calling \Drupal\drupalmoduleupgrader\Plugin\DMU\Fixer\NodeCollectorTrait::getObjects() and getting back a \Pharborist\Node rather than a \Pharborist\NodeCollection. That in turn seems to be down to \Drupal\drupalmoduleupgrader\IndexerInterface::get() returning a NodeCollection with some indexers and an individual Node with others (at least with \Drupal\drupalmoduleupgrader\Plugin\DMU\Indexer\Functions::get()).

All usages of getObjects() immediately iterate the result, so it feels like it could be corrected by ensuring we return a NodeCollection. However I don't see how it was ever working, I don't see any obvious changes in the affected code that caused it to start failing, and if I revert the module back to 1.9 (and use the version of jcnventura/pharborist from the same date) I still get the same failure, so I'm nervous I'm missing something here. Also annoyingly I don't know how to get the module to use the delete plugin outside of a test, so I can't do a manual functional test and verify there definitely is an issue with the delete plugin, and it's fixed with the change. I don't see any issues in the queue that might be related.

I've added the possible solution suggested in the previous comment of ensuring we always return a node collection; as mentioned before I don't know how to test it properly with the module and I'm new to it, so careful review appreciated, thanks!