ci: add getchaintxstats ubsan suppressions

Add ubsan suppressions for integer overflows in the getchaintxstats RPC.

getchainstatstx line "int nTxDiff = pindex->nChainTx - past_block.nChainTx" can
trigger ubsan integer overflows when assumeutxo snapshots are loaded, from
subtracting unsigned values and assigning the result to a signed int.

The overflow behavior probably exists in current code but is hard to trigger
because it would require calling getchainstatstx at the right time with
specific parameters as background blocks are being downloaded. But the overflow
behavior becomes easier to trigger in the upcoming commit removing fake
nChainTx values, so a suppression needs to be added before then for CI to pass.

getchainstatstx should probably be improved separately in another PR to not
need this suppression, and handle edge cases and missing nChainTx values more
carefully.

test/sanitizer_suppressions/ubsan

@@ -51,6 +51,7 @@ unsigned-integer-overflow:CCoinsViewCache::Uncache
 unsigned-integer-overflow:CompressAmount
 unsigned-integer-overflow:DecompressAmount
 unsigned-integer-overflow:crypto/
+unsigned-integer-overflow:getchaintxstats*
 unsigned-integer-overflow:MurmurHash3
 unsigned-integer-overflow:CBlockPolicyEstimator::processBlockTx
 unsigned-integer-overflow:TxConfirmStats::EstimateMedianVal
@@ -61,6 +62,7 @@ implicit-integer-sign-change:CBlockPolicyEstimator::processBlockTx
 implicit-integer-sign-change:SetStdinEcho
 implicit-integer-sign-change:compressor.h
 implicit-integer-sign-change:crypto/
+implicit-integer-sign-change:getchaintxstats*
 implicit-integer-sign-change:TxConfirmStats::removeTx
 implicit-integer-sign-change:prevector.h
 implicit-integer-sign-change:verify_flags