testBusinessTemplateTwoFileExport: Make output_encoding explicit as it should have always been.

This fixes a failure due to Zope4 now defaulting to utf-8.
14 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:42:17

failed ERP5.PerformanceTest-Master

00:26:16

failed ERP5.UnitTest-Master

01:09:13

failed ERP5.UnitTest-Master.Medusa

01:20:05

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:38

failed ERP5.UnitTest-Master

01:27:47

failed ERP5.UnitTest-Master

01:10:15

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:04

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:53

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:42

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:38

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:33

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:53