Fix test flakiness in ddl_high_priority

Summary:
The ddl_high_priority test occassionally fails with extra output
showing connections remaining over from the previous subtest.
Add a session count to ensure they are cleaned up before the
subtest ends.

Test Plan:
mtr main.ddl_high_priority
mtr rocksdb.ddl_high_priority

Reviewers: aliaksei, mung, rtm

Reviewed By: aliaksei, rtm

Subscribers: webscalesql-eng@fb.com

Differential Revision: https://phabricator.intern.facebook.com/D6436736

Tasks: T23950282

Signature: 6436736:1511976587:2ec22d5349a499cde26f1ac21b90abea8112d801

mysql-test/include/ddl_high_priority_module.inc

@@ -50,6 +50,9 @@
 ## Setup
 ##
 
+# Save the initial number of concurrent sessions
+--source include/count_sessions.inc
+
 connection default;
 
 # create con1
@@ -139,3 +142,5 @@ connection default;
 show processlist;
 
 disconnect con1;
+
+--source include/wait_until_count_sessions.inc