arb_blend_func_extended: Test dual src blending without second color output

Using fragment shader without second color output should not hang gpu when dual source blending is enabled.
It hanged Intel gen8+ GPUs when discarding fragments and depth test being enabled.
There is also a safeguard against lack of second color output in radeonsi.

v2: by Ilia Mirkin
- Set supports_gl_compat_version = 30
- Moved drawing to piglit_display
- Change drawing sequence to: clear red -> draw -> clear green

v3: by Dylan Baker
- Removed run_concurrent=False

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