unnest all columns from a given table
You can use JSON functions to do that for you:

select x.*
from mytest, jsonb_each_text(to_jsonb(mytest)) as x("key",value);


With your sample data, this returns:

key   | value
------+------
col_1 | a1   
col_2 | b1   
col_1 | a2   
col_2 | b2   
col_1 | a3   
col_2 | b3   


to_jsonb(mytest) converts the whole row into a JSON value using the column names as keys and jsonb_each_text() is the unnesting of that JSON value. 

Online example: https://rextester.com/DSNOIU44927