resolve two unused warnings in libtester that show up only in release builds

[spoonincode]

clang18:

/home/spoon/f/leap/src/libraries/testing/tester.cpp:378:25: warning: unused variable '[block, id]' [-Wunused-variable]
  378 |             const auto& [block, id] = t;
      |                         ^
/home/spoon/f/leap/src/libraries/testing/tester.cpp:377:106: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
  377 |       [[maybe_unused]] auto accepted_block_header_connection = control->accepted_block_header().connect([this](const block_signal_params& t) {
      |                                                                                                          ^~~~

an alternative would be to wrap the 5 lines of this connection in a #ifndef NDEBUG

[ericpassmore]

Note:start
category: Tests
component: Internal
summary: Resolve unused warning in libtester.
Note:end