Merge #16294: qt: test: Create at most one testing setup

faa1e0fb17 qt: test: Create at most one testing setup (MarcoFalke)

Pull request description:

  It is assumed that ideally only one BasicTestingSetup exists at any point in time for each process (due to use of globals).

  This assumption is violated in the GUI tests, as a testing setup is created as the first step of the `main` function and then (sometimes) another one for the following test cases.

  So, the gui tests create two testing setups:
  * `BasicTestingSetup` in `main` (added in fa4a04a5a9)
  * a testing setup for individual test cases

  Avoid that by destructing the testing setup in main after creation and then move the explicit `ECC_Stop` to the only places where it is needed (before and after `apptests`).

ACKs for top commit:
  laanwj:
    code review ACK faa1e0fb17

Tree-SHA512: b8edceb7e2a8749e1de3ea80bc20b6fb7d4390bf366bb9817206ada3dc8669a91416f4803c22a0e6c636c514e0c858dcfe04523221f8851b10deaf472f107d82

src/test/setup_common.cpp

@@ -73,7 +73,6 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
     const CChainParams& chainparams = Params();
     // Ideally we'd move all the RPC tests to the functional testing framework
     // instead of unit tests, but for now we need these here.
-
     RegisterAllCoreRPCCommands(tableRPC);
 
     // We have to run a scheduler thread to prevent ActivateBestChain