override sysvars from accounts list

[2501babe]

this is a convenience to make porting stake_instruction.rs tests (and other similar InvokeContext-based tests) much simpler. sysvars from the accounts list are used to override the sysvars on Mollusk, without mutating it. this is to accord with the way existing tests are structured, so we dont need to add any setup logic that doesnt already exist. in general they create sysvar states and pass them into InvokeContext via process_instruction directly

i considered letting it mutate Sysvars but this seems cleaner, especially since we will be able to remove it someday when the bpf programs use sysvar syscalls and dont need to accept sysvar accounts at all

[buffalojoec]

I think this manages to accomplish what you're after here: #64

Let me know if there's more we need!