test: replace usage of at() matcher with alternatives

The PHPUnit at() matcher, used to determine the order that methods are called
on test doubles, has been deprecated in PHPUnit 9 and has been removed in
PHPUnit 10.
Migrate usage of at() to other constructs following Drupal core examples in:
<https://www.drupal.org/node/3218874>

tests/units/Model/ProjectFileTest.php

@@ -148,14 +148,12 @@ class ProjectFileTest extends Base
             ->method('generateThumbnailFromFile');
 
         $this->container['objectStorage']
-            ->expects($this->at(0))
+            ->expects($this->exactly(2))
             ->method('moveUploadedFile')
-            ->with($this->equalTo('/tmp/phpYzdqkD'), $this->anything());
-
-        $this->container['objectStorage']
-            ->expects($this->at(1))
-            ->method('moveUploadedFile')
-            ->with($this->equalTo('/tmp/phpeEwEWG'), $this->anything());
+            ->withConsecutive(
+                ['/tmp/phpYzdqkD'],
+                ['/tmp/phpeEwEWG'],
+            );
 
         $this->assertTrue($fileModel->uploadFiles(1, $files));
 
@@ -232,7 +230,7 @@ class ProjectFileTest extends Base
         );
 
         $this->container['objectStorage']
-            ->expects($this->at(0))
+            ->expects($this->once())
             ->method('moveUploadedFile')
             ->with($this->equalTo('/tmp/phpYzdqkD'), $this->anything())
             ->will($this->throwException(new \Kanboard\Core\ObjectStorage\ObjectStorageException('test')));