PostgreSQL: How to check if the value is in the list of values within the text

Question

I have a single value in one column and a string of values in another column, so how it is possible to check if a single value is in the list of values? The goal is to find an exact value, i.e. only 'L', but not 'L' as part of 'XL'

with cte as
(
select 'L' as test_char, 'S, M, L, XL, XXL, 3XL' as test_column
union
select 'L', '30, 40, 50, 60, 70'
union
select 'L', '30L, 40X, 50M, 60XL, 70XXL'
)   
select test_char, 
       test_column,
       case when test_char in (test_column) then 'yes' else 'no' end as check_column
from cte

I try to check with in but can not find it. (in the third row the check_column need to be 'yes').

it is possible to convert values in strings to an array and to check each value in the array? Or any other ideas?