MDEV-16944 Fix file sharing issues on Windows in mysqltest

On Windows systems, occurrences of ERROR_SHARING_VIOLATION due to
conflicting share modes between processes accessing the same file can
result in CreateFile failures.

mysys' my_open() already incorporates a workaround by implementing
wait/retry logic on Windows.

But this does not help if files are opened using shell redirection like
mysqltest traditionally did it, i.e via

--echo exec "some text" > output_file

In such cases, it is cmd.exe, that opens the output_file, and it
won't do any sharing-violation retries.

This commit addresses the issue by introducing a new built-in command,
'write_line', in mysqltest. This new command serves as a brief alternative
to 'write_file', with a single line output, that also resolves variables
like "exec" would.

Internally, this command will use my_open(), and therefore retry-on-error
logic.

Hopefully this will eliminate the very sporadic "can't open file because
it is used by another process" error on CI.

mysql-test/suite/binlog_encryption/restart_server.inc

@@ -20,7 +20,7 @@
 
 --connection $_cur_con
 --enable_reconnect
---exec echo "wait" > $MYSQLTEST_VARDIR/tmp/mysqld.$rpl_server_number.expect
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.$rpl_server_number.expect
 
 shutdown_server;
 
@@ -31,5 +31,5 @@ if ($rpl_server_parameters)
 {
   --let $_rpl_start_server_command= restart:$rpl_server_parameters
 }
---exec echo "$_rpl_start_server_command" > $MYSQLTEST_VARDIR/tmp/mysqld.$rpl_server_number.expect
+--write_line "$_rpl_start_server_command" $MYSQLTEST_VARDIR/tmp/mysqld.$rpl_server_number.expect
 --source include/wait_until_connected_again.inc