Tiny speedup when listing tags

Noticed that the code was originally concatenating strings just to
then append the result to another string. Now it does not create
temporaries and also preallocates the string buffer.

include/internal/catch_list.cpp

@@ -84,9 +84,18 @@ namespace Catch {
     }
 
     std::string TagInfo::all() const {
-        std::string out;
+        size_t size = 0;
-        for( auto const& spelling : spellings )
+        for (auto const& spelling : spellings) {
-            out += "[" + spelling + "]";
+            // Add 2 for the brackes
+            size += spelling.size() + 2;
+        }
+
+        std::string out; out.reserve(size);
+        for (auto const& spelling : spellings) {
+            out += '[';
+            out += spelling;
+            out += ']';
+        }
         return out;
     }