arb_blend_func_extended: Add test checking corruption on BSW/BDW
Test exercises a bug on BSW/BDW Intel platforms originally found in skia tests.
- Draw discarding some pixels
- Enable dual source blending
- Draw with shader without discards using dual src blending
- As a result some pixels in the region of the first draw may be corrupted
One iteration may not reproduce the issue; with current value of 100 iterations the test didn't pass a single time running in a loop for more than an hour.