There might be a case when identical nonces are generated for tx6, tx7 or
tx8 (they are not in mempool, so each of them pass mempool-presence
check). In this case test fails due to the lack of hashes into mp.conflicts
map (two of tx6, tx7 or tx8 have identical hashes) with the following
error:
```
=== RUN TestMempoolAddRemoveConflicts
--- FAIL: TestMempoolAddRemoveConflicts (0.00s)
mem_pool_test.go:376:
Error Trace: mem_pool_test.go:376
Error: Not equal:
expected: 4
actual : 3
Test: TestMempoolAddRemoveConflicts
```
Fixed by maling the nonce non-random.
They follow C# conversion rules, but differ from our `bigint` module
conversions:
1. String must be big-endian.
2. Sign extension is 4-bit in size (single hex character)
and not 8-byte.
