logging: fix color being shown even if disabled

If CONFIG_LOG_BACKEND_SHOW_COLOR is disabled, error and warning
lines are still being outputted with color, which is contrary to
what the Kconfig says. So change the color settings for error
and warning lines to no color if this Kconfig is disabled.

The log_output test is also amended to account for this.
Though the test is not skipped as no color is still some colors
(... well... maybe).

Fixes #77150

Signed-off-by: Daniel Leung <daniel.leung@intel.com>