Merge of this test from trunk to 5.5 to have a successful weekly test on Windows.

mysql-test/r/variables-big.result

+SET @def_var= @@session.transaction_prealloc_size;
 SET SESSION transaction_prealloc_size=1024*1024*1024*1;
 SHOW PROCESSLIST;
 Id	User	Host	db	Command	Time	State	Info
@@ -18,3 +19,4 @@ SET SESSION transaction_prealloc_size=1024*1024*1024*5;
 SHOW PROCESSLIST;
 Id	User	Host	db	Command	Time	State	Info
 <Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
+SET @@session.transaction_prealloc_size= @def_var;

mysql-test/t/variables-big.test

@@ -34,6 +34,7 @@
 # There is a significant probablitity that this tests fails with testcase
 # timeout if the testing box is not powerful enough.
 #
+SET @def_var= @@session.transaction_prealloc_size;
 
 --disable_warnings
 SET SESSION transaction_prealloc_size=1024*1024*1024*1;
@@ -53,3 +54,5 @@ SET SESSION transaction_prealloc_size=1024*1024*1024*5;
 SHOW PROCESSLIST;
 --enable_warnings
 
+SET @@session.transaction_prealloc_size= @def_var;
+