Unsure if this is intended or not, but the given command:

execute unless score #Test test matches 1 run function test:test

will never fire if the test scoreboard does not exist. This does not seem consistent with the fact that uninitialized scores for entities/players/fake players are treated as a success and will run the function command.

so the same command above will work if you run:

scoreboard objectives add test dummy

without initializing the #Test score.

I believe this to be inconsistent because if the scoreboard doesn't exist, neither does the score.
if A doesn't have a score of X in B.... B doesn't exist -> it doesn't have the score, should succeed. -FVbico

Further note, also unsure if intended or not:

execute store success score #Test test run scoreboard objectives add test dummy

This will fail completely if the scoreboard does not exist with the error code Unknown scoreboard objective 'test', but will return 0 if the scoreboard does exist.