EVM runtime: Tracking for precompile unit tests

[mriise]

• [x] ec_recover https://github.com/filecoin-project/builtin-actors/pull/654
• [x] sha256 https://github.com/filecoin-project/builtin-actors/pull/615
• [x] ripemd160 https://github.com/filecoin-project/builtin-actors/pull/651
• [x] identity (this is just Vec::from(input), no need for a test)
• [x] modexp https://github.com/filecoin-project/builtin-actors/pull/654
• [x] ec_add https://github.com/filecoin-project/builtin-actors/pull/564
• [x] ec_mul https://github.com/filecoin-project/builtin-actors/pull/564
• [x] ec_pairing https://github.com/filecoin-project/builtin-actors/pull/654
• [x] blake2f https://github.com/filecoin-project/builtin-actors/pull/564