gpgscm: Merge 'opexe_4'.

* tests/gpgscm/scheme.c (opexe_4): Merge into 'opexe_0'.
* tests/gpgscm/opdefines.h: Adapt.

Having separate functions to execute opcodes reduces our ability to
thread the code and prevents the dispatch_table from being moved to
rodata.

• Signed-off-by: Justus Winter <justus@g10code.com>