I fail to match the standard output of a posix shell function with the text I want to test it against

I want to test if the output of a function matches the text I envision it should write to standard output. But the code below never prints

PASS

and I can't figure out why. I also tried putting various

\n

characters or deleting them in the variable

but that didn't help.

error_out()
{
    printf "%s\n" "${ERROR}"
    printf "Return to exit\n"
    read -r THROW_AWAY
    exit
}

ERROR="Test output."
OUTPUT_TO_TEST=$(error_out)
GROUND_TRUTH="Test output.\nReturn to exit\n"
{ [ "${OUTPUT_TO_TEST}" = "${GROUND_TRUTH}" ] \
         && printf "error_out test PASS\n"; } \
    || { \
        printf "error_out test FAIL\n"; \
    }