test/encoding: verify that e.what() starts with expected str

test/encoding: verify that e.what() starts with expected str

boost changes the way how it prints boost::system::system_error in
boost 1.79 -- it appends the stringified error_category at end of
exception::what(), and our buffer::malformed_input is a subclass
of boost::system::system_error.

so we cannot just compare the return value of what() with the
expected string, to be more future proof, let's check if i
starts with the expected string instead.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>