test_setChainParams incorrectly initializes precompiles

[gumb0]

It calls ClientTest::setChainParams which calls ChainParams::loadConfig which first creates a copy of current params then applies values from json on top of it. This includes a call to jsonToAccountMap, which uses std::unordered_map::insert to initialize precompiled contracts. insert doesn't overwrite previous precompiles, when the key already exists.

This way setChainParams keeps the precompiles that were defined in the first used chain params forever.

I think loadConfig should better initialize params starting from default values (including empty precompile map) instead of starting from currently stored values.

This affects running tests via retesteth (possibly should fix at least some of the currently failing ones).