Extensive MODEXP testing WIP

[OlivierBBB]

We need to write tests for MODEXP that will test the various combinations of the following:

Tiny call data: have a `CDS` parameter of
0,
19, 32 + 19, 64 + 19,
32 - 1, 64 - 1, 96 - 1, 

// 19 is just some fixed value

32 * x + 19 cases: make it so that the call data contains NONZERO bytes after the last call data byte
32 * x - 1  cases: make it so that the final byte is 0x02 and the following bytes are random nonzero values

The purpose of the "-1" case is to test for the legal value 0x200 = 512 of one of the bbs, ebs, mbs values

These tests will lead to know MODEXP computation, however one will still pay its execution and replace the return data of the current frame.