refactor: add self-assign checks to classes which violate the clang-tidy check

Both of these cases appear to be harmless, but adding the tests allows us to
turn on the aggressive clang-tidy checks.

src/arith_uint256.h

@@ -43,8 +43,10 @@ public:
 
     base_uint& operator=(const base_uint& b)
     {
+        if (this != &b) {
             for (int i = 0; i < WIDTH; i++)
                 pn[i] = b.pn[i];
+        }
         return *this;
     }
 

src/key.h

@@ -75,6 +75,7 @@ public:
 
     CKey& operator=(const CKey& other)
     {
+        if (this != &other) {
             if (other.keydata) {
                 MakeKeyData();
                 *keydata = *other.keydata;
@@ -82,6 +83,7 @@ public:
                 ClearKeyData();
             }
             fCompressed = other.fCompressed;
+        }
         return *this;
     }
 

src/test/util_tests.cpp

@@ -1508,8 +1508,10 @@ struct Tracker
     Tracker(Tracker&& t) noexcept : origin(t.origin), copies(t.copies) {}
     Tracker& operator=(const Tracker& t) noexcept
     {
+        if (this != &t) {
             origin = t.origin;
             copies = t.copies + 1;
+        }
         return *this;
     }
 };