test: Silence MSVC warnings

test: Silence MSVC warnings

MSVC does not notice non-returning functions (abort() / assert(0))
and warns about paths which end with them in non-void functions:

c:\cygwin\home\ranma42\code\fdo\pixman\test\fetch-test.c(114) :
warning C4715: 'reader' : not all control paths return a value
c:\cygwin\home\ranma42\code\fdo\pixman\test\stress-test.c(133) :
warning C4715: 'real_reader' : not all control paths return a value
c:\cygwin\home\ranma42\code\fdo\pixman\test\composite.c(431) :
warning C4715: 'calc_op' : not all control paths return a value

These warnings can be silenced by adding a return after the
termination call.