i965/vec4: use an envvar to decide to print the assembly on cmod_propagation tests

i965/vec4: use an envvar to decide to print the assembly on cmod_propagation tests

The complete way to do this would be parse INTEL_DEBUG and
print the output if DEBUG_VS (or a new one) is present
(see intel_debug.c).

But that seems like an overkill for the unit tests, that
after all, the most common use case is being run when
calling make check.

v2: use the same idea for the fs counterpart too, as suggested by
    Matt Turner

Reviewed-by: Matt Turner <mattst88@gmail.com>