Add missing TLS tests

While working on #483 (closed), a number of test gaps have been identified. Due to @godfryd's unexpected unavailability and approaching release date, @marcin and @tomek decided that the best way forward is to make an exception to the requirement for unit tests, merge #483 (closed) and implement the tests soon afterward. Hence this ticket.

The following tests are needed:

• extensions to PingMachine code in backend/server/restservice/machines.go:437, see !267 (comment 194921)
• extensions to UpdateMachine code in backend/server/restservice/machines.go:536, see !267 (comment 194922)
• better testing for DEB/RPM packages download, as suggested in !267 (comment 194941)
• machines-page component received substantial updates, but there are no unit tests to cover this functionality, see !267 (comment 194944)