tests: save mysqldump atomically with runUnitTest --save

The pattern of using > will start by erasing the file, so if mysqldump
does not run to completion developer will loose the previous dump, but
in such case, that previous dump was better than nothing.

product/ERP5Type/tests/custom_zodb.py

@@ -40,10 +40,11 @@ if save_mysql:
     # The output of mysqldump needs to merge many lines at a time
     # for performance reasons (merging lines is at most 10 times
     # faster, so this produce somewhat not nice to read sql
-    command = 'mysqldump %s > %s' % (getMySQLArguments(), dump_sql_path,)
+    command = 'mysqldump %s > %s.tmp' % (getMySQLArguments(), dump_sql_path,)
     if verbosity:
       _print('Dumping MySQL database with %s ...' % command)
     subprocess.check_call(command, shell=True)
+    os.rename(dump_sql_path + '.tmp', dump_sql_path)
 
 if load:
   if save_mysql: