Wait for ProgramTest to support overwriting builtins with BPF programs

[buffalojoec]

Problem

The runtime only recently was modified to allow builtin programs to be overwritten (#).

In order to use solana-program-test to test Core BPF versions of builtin programs, we'll need to wait
until this change is made available.

Until then, we have to use an arbitrary program address, which can make testing both inaccurate and
annoying.

Solution

Wait for the next release to use the updated runtime through solana-program-test.

[buffalojoec]

Looks like we might need to take a closer look at this one. It only works if you set prefer_bpf to false.

[buffalojoec]

Closing in favor of #16.