Turns out we needed more comprehensive testing; we ended up with three
separate tests. To avoid changing test_channel_drainage as we fix
spendable_msat, I substituted raw numbers there.
The first is a variation of the existing tests, testing we can't
exceed spendable_msat, and we can pay it, both ways.
The second is with a larger amount, which triggers a different problem.
The final is with a giant channel, which tests our 2^32-1 msat cap.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>