fix issue with mutation in test generation

[djrtwo]

Mutation issues in spec generators.

next_slots_with_attestations creates a copy of the state, but _transition_until_active assumes that the original state passed in is mutated with the transition. This means that post is yielded incorrectly due to no affects on state from _transition_until_active.

This just returns the mutated state to be used for the yield.

Note, we have a pretty bad mismatch between some transition functions mutating and some copying and returning the new state. This PR does not address this fundamental issue. just fixes a discrete test generation error.

[protolambda]

LGTM, this fixes the issue in test running the spec tests in ZRNT.

For context, previous testing problem showed that the yielded post state was not the last, but some state before transition of all blocks:

proto transition_with_deposit_right_after_fork (master *+)$ for i in {0..26}; do zcli pretty altair SignedBeaconBlock --preset-phase0=minimal --preset-altair=minimal ssz_snappy:blocks_${i}.ssz_snappy | grep state_root ; done

    "state_root": "0x85bd6b2f1c2200078d799f547c81adb0eef92ff7b0ccf07119da4df4c64f5ae6",
    "state_root": "0xa123a4cc5d666272a981892b10b2e4a10bc98d56147b5f265df94845b628fb46",
    "state_root": "0xd9ef8bc058b7821caf028677c849d6cac4bed01880dd9288c49a849efbf371c5",
    "state_root": "0xbb4ef61f4150843737b48c7e404c4d16615e3f0f6da453a99060f5d1d7bf146a",
    "state_root": "0xcd7d51a19ce75d8759387817bab377e39d7d960eacdc139a8646580c7a0ecd77",
    "state_root": "0x11ebac78a8e00cb7f4a1873bc0c2bc1310d09471f44618739f4a85cfdaabf2cc",
    "state_root": "0x51b032de065a3b0e8fa6e623c9b8f4b51ff1b70749cb261c6edce2142605128d",
    "state_root": "0xd336f3635684628880904c25d3fe1932c034b8779a87998dfd2cbabd998ddbdb",
    "state_root": "0x28c73560282fbdb40e58c44d5e3d547e5b76f3803e74c4bc93cac0590bb1a435",
    "state_root": "0x6bf79cca785616ae3185fb9eecfc8ec02ff6ff1fabb0568cb798c2e0ea914edf",
    "state_root": "0x1e5b0a6faa50fa7c2d7960927d8e3bbeee9aec20bb7a1374547afb087a4ad26b",
    "state_root": "0x4c2c2888a11e24f617a31f3120ab556d78ce9c04715ad6385bc959047c2b36d9",
    "state_root": "0xa55ce61e6c02346623472cfbcb39ce19fbb0eca9683c369f9bd97bcd7740d879",
    "state_root": "0x574e151b06eaa62c9b8d35df099ea5f6fd4220e58afa3c74ae64ac81627c825a",
    "state_root": "0x9c9f10a298338354cf412e6c82788e78466fe9ed78a4709f3004d7f69e19d0f0",
    "state_root": "0x5241fefbd11aa0ed94ce513edcfcb88b48b9a7ff8c2a4add6b5b9c59363f247e",
    "state_root": "0xd6cbf42139434e80ca2658ef29eca555997d38fda7a2e7c4aff0f01c91fce0f8",
    "state_root": "0xea06c1d5bd0e126339a46981d105ebce47aa4f4c7334e41cd6a652489924e912",
    "state_root": "0x0c057e2e8f2d3edd9aaf861a1fefd7ef8d5937e13d8168219bd126b872a5f5cd",
    "state_root": "0xb8210083437b178deb8db8a84075a4f3a2a8ad22d955bf1217a5599b347d7c28",
    "state_root": "0x33ee0d2b502ddece42782f577292f791546e528c28e7859957cce10045c37afc",
    "state_root": "0x5f79dc3783f62c0ff80e94cf1cbdd8b91163c2d9c1cb0ca5dc1b2a9b0543ed42",
    "state_root": "0x4ffeecb085152f2ab502f665a28487c9e27ca61e9c6d2e2cc52ea96ccf3bafd4",
    "state_root": "0x28f82e79d51882758a3c4679fb1c5921f7754fe9e3a09fa05889a4b28cdd42a5",
    "state_root": "0xe00ae2bfd04bf6ccbdb06fc510dbead2ccd3f3a01b532d6f406782a3c5139fea",
    "state_root": "0xe88ea4066362151c507cfc5059411f84f72cb02c2f0a9943c126234eb67afb16",
    "state_root": "0xea1f6676ebfb262218f20022d18ccc7da004900e3d0353e8fb58c96b8eb2ccef",

proto transition_with_deposit_right_after_fork (master *+)$ zcli root altair BeaconState --preset-phase0=minimal --preset-altair=minimal ssz_snappy:post.ssz_snappy

0x28c73560282fbdb40e58c44d5e3d547e5b76f3803e74c4bc93cac0590bb1a435