Problem:
After #40270, events are no longer emitted from the automatic background
detection. This applies not just during startup, but also if the user
manually changes the background of their terminal.
Solution:
Set the background as normal, assuming that a normal terminal will
respond within 100 ms. Change test to match expected behavior:
- BG set during startup won't trigger user autocmds since it runs before
any user config
- If the terminal takes longer than 100 ms to respond to initial OSC 11,
it does trigger the OptionSet, but it is triggered through the normal
path to ensure values like v:option_new are set #38551
- BG change after startup still triggers autocmds #41146
8d406ed2ac