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/util/threadinterrupt.cpp

@@ -9,11 +9,16 @@
 
 CThreadInterrupt::CThreadInterrupt() : flag(false) {}
 
-CThreadInterrupt::operator bool() const
+bool CThreadInterrupt::interrupted() const
 {
     return flag.load(std::memory_order_acquire);
 }
 
+CThreadInterrupt::operator bool() const
+{
+    return interrupted();
+}
+
 void CThreadInterrupt::reset()
 {
     flag.store(false, std::memory_order_release);