New tests to check that, with separable program objects, interface
matching by name or location are still checked at link time between
the inner interfaces existing inside the program.

From the ARB_separate_shader_objects spec v.25:

  " With separable program objects, interfaces between shader stages
    may involve the outputs from one program object and the inputs
    from a second program object.  For such interfaces, it is not
    possible to detect mismatches at link time, because the programs
    are linked separately.  When each such program is linked, all
    inputs or outputs interfacing with another program stage are
    treated as active.  The linker will generate an executable that
    assumes the presence of a compatible program on the other side of
    the interface.  If a mismatch between programs occurs, no GL error
    will be generated, but some or all of the inputs on the interface
    will be undefined."

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>