MDEV-7329 plugins.pam_cleartext fails sporadically in buildbot

wait until the failed connection thread completely dies
before uninstalling pam plugin

mysql-test/suite/plugins/t/pam.test

@@ -29,5 +29,6 @@ EOF
 --remove_file $MYSQLTEST_VARDIR/tmp/pam_bad.txt
 drop user test_pam;
 drop user pam_test;
+let $count_sessions= 1;
+--source include/wait_until_count_sessions.inc
 uninstall plugin pam;
-

mysql-test/suite/plugins/t/pam_cleartext.test

@@ -19,4 +19,6 @@ EOF
 
 drop user test_pam;
 drop user pam_test;
+let $count_sessions= 1;
+--source include/wait_until_count_sessions.inc
 uninstall plugin pam;