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.

Reviewed By: asandryh, Tema

Differential Revision: D6436736

fbshipit-source-id: 8a8e957

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