tests: minor fixes for backup and merge modules

Author: gsmolk

tests/backup.py

@@ -5,6 +5,7 @@
 import shutil
 from distutils.dir_util import copy_tree
 from testgres import ProcessType
+import subprocess
 
 
 module_name = 'backup'

tests/merge.py

@@ -7,6 +7,7 @@
 import shutil
 from datetime import datetime, timedelta
 import time
+import subprocess
 
 module_name = "merge"