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/innodb/t/innodb-change-buffer-recovery.test

@@ -51,7 +51,7 @@ DELETE FROM t1 WHERE a=1;
 INSERT INTO t1 VALUES(1,'X',1);
 
 SET DEBUG_DBUG='+d,crash_after_log_ibuf_upd_inplace';
---exec echo "wait" > $_expect_file_name
+--write_line wait $_expect_file_name
 --error 2013
 # This should force a change buffer merge
 SELECT b FROM t1 LIMIT 3;