Split channel_reserve_test so we don't rely on unfilled Err actions

Split channel_reserve_test so we don't rely on unfilled Err actions

Currently channel_reserve_test sends a garbage update_add_htlc
message and then relies on it being silently ignored to continue
using the channel. This shouldn't be the case, so take the easy
way out and split the test in two, at first not delivering the
bogus update_add_htlc and then delivering it, but not running the
rest of the test.