test: expose the failure of batch verification on 4 proofs and so on

[garyyu]

Enhance the test to expose the failure of batch verification on range proofs.

So far, very confusing problem:

• Mac OS OK, no this problem
• On Linux, always fail for 4/17/18/19/20 range proofs verification, no other failure up to 1000 range proofs
• If rollback https://github.com/mimblewimble/rust-secp256k1-zkp/commit/c61bc85011ee48dce393e6db0907be4637291539 (which is forchange proof validate to be non-consensus breaking), then on Linux, always fail for 4 range proofs verification, only fail for 4!

Investigation still on going.

And this PR is just to enhance the test to expose the problem, and surely it will cause travis-ci fail on Linux test, but it's the purpose.

[garyyu]

CI test fail as expected:)

---- pedersen::tests::test_bullet_proof_verify_multi stdout ----
1
2
3
4
5
6
7
4 proofs batch verify failed
17 proofs batch verify failed
18 proofs batch verify failed
19 proofs batch verify failed
20 proofs batch verify failed
99thread 'pedersen::tests::test_bullet_proof_verify_multi' panicked at 'assertion failed: `(left == right)`
  left: `5`,
 right: `0`', src/pedersen.rs:1223:3
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
    pedersen::tests::test_bullet_proof_verify_multi
test result: FAILED. 46 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out