Bug#41889: Test main.innodb_bug38231 is failing w/embedded server (server crash)

The problem is that a mysql connection instance is not thread-safe
and reentrant, meaning that it can't be used concurrently and can't
be re-entered while it's already running. This applies for any form
of the server (embedded or not), but this rule can be violated in a
test case if the test sends a new command without waiting for the
result of previous command that was sent asynchronously and this can
lead to hangs when over a network or to crashes under embedded server
as the server query execution path will be re-entered concurrently
with the same connection structure.

The solution is to rework the test case so that the aforementioned
rule is obeyed.

mysql-test/t/innodb_bug38231.test

@@ -16,7 +16,6 @@ CREATE TABLE bug38231 (a INT);
 
 -- connect (con1,localhost,root,,)
 -- connect (con2,localhost,root,,)
--- connect (con3,localhost,root,,)
 
 -- connection con1
 SET autocommit=0;
@@ -27,11 +26,6 @@ SET autocommit=0;
 -- send
 LOCK TABLE bug38231 WRITE;
 
--- connection con3
-SET autocommit=0;
--- send
-LOCK TABLE bug38231 WRITE;
-
 -- connection default
 -- send
 TRUNCATE TABLE bug38231;
@@ -39,6 +33,7 @@ TRUNCATE TABLE bug38231;
 -- connection con1
 # give time to TRUNCATE and others to be executed; without sleep, sometimes
 # UNLOCK executes before TRUNCATE
+# TODO: Replace with wait_condition once possible under embedded server.
 -- sleep 0.2
 # this crashes the server if the bug is present
 UNLOCK TABLES;
@@ -46,16 +41,13 @@ UNLOCK TABLES;
 # clean up
 
 -- connection con2
-UNLOCK TABLES;
-
--- connection con3
+-- reap
 UNLOCK TABLES;
 
 -- connection default
-
+-- reap
 -- disconnect con1
 -- disconnect con2
--- disconnect con3
 
 # test that TRUNCATE works with with row-level locks