Hello,
We do allow you to configure virtual directories with our sftp server. Under the SFTP Users page (or SFTP Groups, or SFTP IPs) you can create a user or use the Default configuration. On this page, click the Virtual Directories radio button, then assign your virtual directory test1 to the path d:\test1.
At this point to restrict the user from seeing any other subdirectories, you would use the NTFS file system to restrict the user from any other subdirectories other than the directory you want them to have access. For example, you would deny them access to the testob folder, and grant them read/list access to the testap directory.