fixed `tests/authorize.sh' assertions

A change in the error message for wrong shared secrets broke half of the
`authorize.sh' tests. I've introduced a new `test_it_first_line_fail'
function to handle multi-line response bodies that only checks the first
line (on top of the exit status) as the name implies.
s/AM_CONFIG_HEADER/AC_CONFIG_HEADERS/

According to rpmlint(1), AM_CONFIG_HEADER is deprecated.
make the rpm own the agent_secret file