Valgrind: fix "definitely lost" memory in test - test-lots

==67054== 320 bytes in 32 blocks are definitely lost in loss record 2,213 of 2,238
==67054==    at 0x4848A13: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==67054==    by 0x503C550: g_malloc0 (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7600.1)
==67054==    by 0x118AD8: get_random_transaction_with_currency (test-engine-stuff.cpp:1395)
==67054==    by 0x119897: add_random_transactions_to_book (test-engine-stuff.cpp:1848)
==67054==    by 0x115FE8: run_test() (test-lots.cpp:99)
==67054==    by 0x1160BF: main (test-lots.cpp:135)
==67054==

libgnucash/engine/test-core/test-engine-stuff.cpp

@@ -52,6 +52,9 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+
+#include <string>
+
 #include <qof.h>
 
 #include "Account.h"
@@ -1376,8 +1379,6 @@ get_random_transaction_with_currency(QofBook *book,
 {
     Transaction* trans;
     KvpFrame *f;
-    gint num;
-    gchar *numstr;
 
     if (!account_list)
     {
@@ -1392,8 +1393,6 @@ get_random_transaction_with_currency(QofBook *book,
         return NULL;
     }
 
-    numstr = g_new0(gchar, 10);
-
     trans = xaccMallocTransaction(book);
 
     xaccTransBeginEdit(trans);
@@ -1402,9 +1401,10 @@ get_random_transaction_with_currency(QofBook *book,
                           currency ? currency :
                           get_random_commodity (book));
 
-    num = get_random_int_in_range (1, max_trans_num);
-    g_snprintf(numstr, 10, "%d", num);
-    xaccTransSetNum(trans, numstr);
+    gint num = get_random_int_in_range (1, max_trans_num);
+    auto numstr = std::to_string(num);
+    xaccTransSetNum(trans, numstr.c_str());
+
     set_tran_random_string_from_array(trans, xaccTransSetDescription,
                                       sane_descriptions);
     trn_add_ran_time(trans, xaccTransSetDatePostedSecs);