Corrected lut mode

[radarhere]

Suggestions for python-pillow#8290

• The first test_correct_mode() lut_mode should be RGBA
• Replaced ("lut_mode", "table_size") with "lut_mode, table_size". It seems simpler to me, and is the style that Pillow is using elsewhere
• The second argument of generate_identity_table() is size, so I've split table_size into table_channels and table_size

[akx]

I somewhat disagree with the parametrize string mode – I think separate parameters are easier to visually grep, and that's what I'm used to :)

But that's fine.

[akx]

@radarhere Btw, according to https://docs.astral.sh/ruff/rules/pytest-parametrize-names-wrong-type/ both styles are being used in Pillow right now. csv style overwhelmingly more, but there are some tuple cases too.

(See: python-pillow#8295)