Hello Alfred, intraclass correlation coefficient (ICC) is the best option. According to the standard of COSMIN the used of ICC to assess is methologicaly better.
You cannot use t test for it. The procedure in test - retest is to find the linear relationship between the scores of first and the second tests using correlation.
Also, please note that one of the most common measures of reliability (internal consistency) of questionnaires is Cronbach's alpha