Add More HRMP Channel Opening Tests (#2915)

A recent referendum failed because the parachain's sovereign account had
enough funds for the deposit (the runtime at the time required a
deposit) but not enough left over for the existential deposit, which
must remain free.

The current logic does not require a deposit for channels with system
chains, and the current tests check that none is taken. This new test
simply ensures that even without any balance in a parachain's sovereign
account, opening a channel with a system chain will be successful.