You can't insert your Account outside your test method and access it directly.
You have two solutions:
- Add a test setup method which inserts the account. Then request this account from your test method
- Insert your account at the start of your test method like this:
static testMethod void testAttachments() {
Account testAccount = new Account();
testAccount.Name = 'REALIANCE';
insert testAccount;
system.debug('line 2');
//system.debug('id Testaccount: '+testAccount.id);
system.debug(testAccount.Id);
Facturatie__c testFactuur = new Facturatie__c();
system.debug('line 3');
system.debug(testAccount.Name);
testFactuur.Accountname__r.Name = testAccount.Name;
system.debug('line 4');
insert testFactuur;
system.debug('id testFactuur: '+testFactuur.id);
system.debug(testAccount.Name); is ok