test: Drop excessive locking in CAddrManTest::SimConnFail

The unit test is single threaded, so there's no need to hold the mutex
between Good() and Attempt().

This change avoids recursive locking in the CAddrMan::Attempt function.

Co-authored-by: John Newbery <john@johnnewbery.com>

src/test/addrman_tests.cpp

@@ -74,9 +74,9 @@ public:
     // Simulates connection failure so that we can test eviction of offline nodes
     void SimConnFail(const CService& addr)
     {
-         LOCK(cs);
          int64_t nLastSuccess = 1;
-         Good_(addr, true, nLastSuccess); // Set last good connection in the deep past.
+         // Set last good connection in the deep past.
+         Good(addr, true, nLastSuccess);
 
          bool count_failure = false;
          int64_t nLastTry = GetAdjustedTime()-61;