* Fix esoteric test config variable
EIP155 canonically (on Ethereum mainnet, for which these test were developed)
happens simultaneously with EIP158/161 and other "Spurious Dragon" upgrades.
This test is intended to test the transition from the EIP158/161 era (Spurious Dragon)
to the Byzantium era, with Byzantium changes happening at 5.
* Add missing 'modexp' precompiled contract
modexp was introduced via EIP198 as a part of the Byzantium fork,
which this test should be testing against.
b6afc81d69