Add test for duplicate rangeproof inflation bug

[trevyn]

All credit to @snape479's work in https://github.com/mimblewimble/grin/pull/3621, cherry-picked on top of #3628.

First two commits are just cherry-picks of c4ab693 & 0354ed3:

git remote add snape479 https://github.com/snape479/grin.git
git fetch snape479
git cherry-pick c4ab693 0354ed3

Verify as failing on v5.0.1:

git remote add snape479 https://github.com/snape479/grin.git
git fetch snape479
git checkout v5.0.1
git cherry-pick c4ab693 0354ed3
cargo test --all -- test_overflow_cached_rangeproof

Completed:

• [x] Conditionally compile output_negative so it can only ever be called from tests
• [x] Verify that antiochp's review comments are addressed from https://github.com/mimblewimble/grin/pull/3621
• [x] Improve validation of InvalidBlockProof type

[phyro]

can confirm the two cherry-picked commits fail on v5.0.1 with

test test_overflow_cached_rangeproof ... FAILED

failures:

---- test_overflow_cached_rangeproof stdout ----
thread 'test_overflow_cached_rangeproof' panicked at 'Negative output accepted into block!', chain/tests/mine_simple_chain.rs:918:17

[antiochp]

Just taking a look locally and will merge shortly. :+1: