test: FakeNodeClock follow-ups in unit tests #35497

Currently, all test cases using TestChain100Setup or a derived class
like BuildChainTestingSetup are using mocktime by default due to the
SetMockTime call in the TestChain100Setup ctor.

This is confusing, because test cases using mocktime explicitly seem to
imply that before they set the mocktime, real time was used.

E.g. index_reorg_crash claimed in a comment to "Enable mock time".

Fix this issue by adding a FakeNodeClock m_clock field to
TestChain100Setup. Then, use the m_clock instead of explicit calls to
SetMockTime or to a (now) shadowing local FakeNodeClock variable.

The context is easier to reason about: E.g.,

* in TestBasicMining it allows to drop manual SetMockTime(0) calls,
* in connections_desirable_service_flags it allows to drop manual calls
  to SetMockTime(GetTime<std::chrono::seconds>() + _n_) and replace them
  by operator+=(_n_)
* in wallet_tests it clarifies that the mocktime does not persist
  outside the AddTx function

This is easier to reason about, because it will automatically take care
of properly setting INITIAL_MOCK_TIME in the ctor. Also, it allows to
drop the ElapseTime and replace it with a call to operator+=()