Apparently you do not need the parentheses, though it could be argued they improve readability.

Concise!

the first test is wrong. Should expect "9401D3", which still is not what the solution gives though.

But you are right, the testbench is bad if it doesnt check for correctly using 2 digits to represent values < 10 (well it probably does test 0 which we here have a hardcoded solution for.. notice how he uses <= instead of < to catch the 0 case)

I was testing your solution and I noticed it does not pass these tests:

Test.assert_equals(rgb(148, 1, 211), '9411D3', 'Noooo') # Expected: "9411D3", instead got: "941D3"

Test.assert_equals(rgb(148, 15, 211), '94FFD3', 'Noooo') # Expected: "94FFD3", instead got: "94FD3"