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/main/plugin_loaderr.test

@@ -13,14 +13,14 @@ FROM INFORMATION_SCHEMA.PLUGINS WHERE plugin_name = 'innodb';
 --echo #
 --echo # MDEV-6351 --plugin=force has no effect for built-in plugins
 --echo #
---exec echo "wait" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--write_line wait $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
 --shutdown_server
 --source include/wait_until_disconnected.inc
 
 --error 1
 --exec $MYSQLD_CMD --innodb=force --innodb-page-size=6000 --disable-log-error
 
---exec echo "restart" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--write_line restart $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
 --enable_reconnect
 --source include/wait_until_connected_again.inc
 --disable_reconnect