phpunit: use $db instead of $this->savdb

The global variables are stored in $this, and in particular $db is used
through this mean. But $this->savdb is supposed to be the immutable
global state that is stored at the test class instantiation and restored
at the beginning of each test.

For $this->savdb, I don't think any consequences are created by this,
since the object is saved by each test anyway and the $db object is
mostly a query object within a transaction, but future change could use
this to inject a different stateful $db object to trace some behaviour
in the test for instance, so make sure the correct one is used.

test/phpunit/CommonInvoiceTest.php

@@ -138,7 +138,7 @@ class CommonInvoiceTest extends PHPUnit\Framework\TestCase
 		$langs=$this->savlangs;
 		$db=$this->savdb;
 
-		$localobject=new Facture($this->savdb);
+		$localobject=new Facture($db);
 		$localobject->fetch(1);
 		$localobject->date = dol_mktime(12, 0, 0, 1, 1, 2010);