always use a 8bit enum for StackVar

correct StackVar types for opcode opSep and opClose in unit test
and test them.

Change-Id: I89fac97d791eb12667ce5218ff31e798c8711e10
Reviewed-on: https://gerrit.libreoffice.org/36000Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>