fuzz: make it possible to mock (fuzz) CThreadInterrupt

* Make the methods of `CThreadInterrupt` virtual and store a pointer to
  it in `CConnman`, thus making it possible to override with a mocked
  instance.
* Initialize `CConnman::m_interrupt_net` from the constructor, making it
  possible for callers to supply mocked version.
* Introduce `FuzzedThreadInterrupt` and `ConsumeThreadInterrupt()` and
  use them in `src/test/fuzz/connman.cpp` and `src/test/fuzz/i2p.cpp`.

This improves the CPU utilization of the `connman` fuzz test.

As a nice side effect, the `std::shared_ptr` used for
`CConnman::m_interrupt_net` resolves the possible lifetime issues with
it (see the removed comment for that variable).

src/test/fuzz/connman.cpp

@@ -13,6 +13,7 @@
 #include <test/fuzz/fuzz.h>
 #include <test/fuzz/util.h>
 #include <test/fuzz/util/net.h>
+#include <test/fuzz/util/threadinterrupt.h>
 #include <test/util/setup_common.h>
 #include <util/translation.h>
 
@@ -71,7 +72,8 @@ FUZZ_TARGET(connman, .init = initialize_connman)
                      addr_man,
                      netgroupman,
                      Params(),
-                     fuzzed_data_provider.ConsumeBool()};
+                     fuzzed_data_provider.ConsumeBool(),
+                     ConsumeThreadInterrupt(fuzzed_data_provider)};
 
     const uint64_t max_outbound_limit{fuzzed_data_provider.ConsumeIntegral<uint64_t>()};
     CConnman::Options options;