gallium/tests: add trivial srgb<->linear conversion test
This would've caught 8829f9cc ("u_format: add ETC2 to util_format_srgb/util_format_linear").
Suggested-by: @anholt in !1652 (comment 207715)
This would've caught 8829f9cc ("u_format: add ETC2 to util_format_srgb/util_format_linear").
Suggested-by: @anholt in !1652 (comment 207715)