Fix compilation warning due to missing base_tester destructor.

[greg7mdp]

base_tester used to have an empty virtual destructor: virtual ~base_tester() {}. Having a destructor disables the implicit move constructor.

My savanna_cluster has tester as a base class of its node. It is convenient because we can call all the tester methods directly.

However, in a recent change as a PR review request, I updated the nodes from the savanna_cluster to be stored in a std::vector instead of a std::array. std::vector requires its elements to have a default and a move constructor.

To address this issue, in the recently merged PR, I removed virtual ~base_tester() {}, which has the side effect of re-enabling the default move constructor, so savanna_cluster compiled fine. However it created compilation warnings in other tests such as:

In file included from /home/greg/github/enf/spring/plugins/producer_plugin/test/test_trx_full.cpp:5:
/home/greg/github/enf/spring/libraries/testing/include/eosio/testing/tester.hpp:564:7: error: constructor for 'eosio::testing::tester' must explicitly initialize the base class 'base_tester' which does not have a default constructor
  564 |       tester(setup_policy policy = setup_policy::full, db_read_mode read_mode = db_read_mode::HEAD, std::optional<uint32_t> genesis_max_inline_action_size = std::optional<uint32_t>{}) {
      |       ^

This PR adds back the explicit empty destructor in base_tester, as well as the two required defaulted constructors. Everything now compiles without tester-related warnings.

[ericpassmore]

Note:start
group: CLEANCODE
category: INTERNALS
summary: Put back empty base_tester destructor for better hygiene.
Note:end