Fix infinite loop in VString::replaceSubstr

If the replaced string was a suffix of the replacement, we used to get
an infinite loop.

src/V3String.h

@@ -143,6 +143,8 @@ public:
     static string aOrAn(const string& word) { return aOrAn(word.c_str()); }
     // Hash the string
     static uint64_t hashMurmur(const string& str) VL_PURE;
+
+    static void selfTest();  // Test this class
 };
 
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