Should a Fact table contain a column with a single unique value?
What I recommend is that you have a fact_source table:

CREATE TABLE fact_source
(
  fact_source_id INTEGER,  -- auto-increment.
  fact_source_name VARCHAR (50)
);


Use your own RDBMS's (you didn't say which) own auto incrementing key system to generate the fact_source_id)

INSERT INTO fact_source fact_source_name) VALUES ('Sales'), ('Testing');


and use that as a FOREIGN KEY in the relevant tables.

You can then "pre-cook" your user-friendly (i.e. text) output in report queries if your testing shows that it works nicely and isn't a resource burden.

You should not use ENUMs for the reasons outlined here. Just because systems have features does not mean that you have to use them! You should always use either CHECK constraints (Unbelievably, MySQL doesn't have these) or a lookup table as described above. 

Storing your key as a SMALLINT (or similar) will save a bit of space!