MDEV-20162: fix connect-abstract test case

The check-testcase record uses a mysqltest connection
to the database to do the recording. With the server configured
as an abstract socket, the mysqltest client cannot connect and
fails.

We work around this by starting the server as normal and then
restart with an abstract socket and test this.

This didn't affect Windows as it just did a tcp connection.
So this did affect all unix socket based systems except Linux
as this was the only one that supported abstract sockets.

mysql-test/main/connect-abstract.cnf

 
 !include include/default_my.cnf
 
-[mysqld.1]
-socket=       @ENV.ABSTRACT_SOCKET
-
 # Using @OPT.port here for uniqueness
 [ENV]
 ABSTRACT_SOCKET= @mtr-test-abstract-socket-@OPT.port

mysql-test/main/connect-abstract.result

-connect con1,localhost,root,,test,,$ABSTRACT_SOCKET;
 select 1;
 1
 1
-disconnect con1;

mysql-test/main/connect-abstract.test

 --source include/linux.inc
 --source include/not_embedded.inc
 
+let $restart_parameters=--socket=$ABSTRACT_SOCKET
+--source include/kill_mysqld.inc
+--source include/start_mysqld.inc
+
 connect(con1,localhost,root,,test,,$ABSTRACT_SOCKET);
 select 1;
-disconnect con1;

mysql-test/unstable-tests

@@ -37,7 +37,6 @@ main.backup_stages                    : MDEV-23401 - Bad file descriptor
 main.binary_to_hex                    : MDEV-20211 - Wrong result
 main.check_constraint                 : Modified in 10.4.18
 main.connect                          : MDEV-17282 - Wrong result
-main.connect-abstract                 : MDEV-20162 - Could not execute 'check-testcase'
 main.connect2                         : MDEV-13885 - Server crash
 main.create                           : Modified in 10.4.18
 main.create_delayed                   : MDEV-10605 - failed with timeout