mount attaches a device node (physical disk, remote nfs mount, etc.) to a directory. What you're trying to do is map one directory to another directory. There are a few ways to handle this.
The most common is called a symbolic link. It essentially creates a filesystem entry that redirects to another location within the filesystem. If you simply want user to be able to access the document root of test.com from inside his home directory, this is the most simple approach.
Alternatively, you can use the
bind mount type. You must be sure that the target directory exists before you use this type, or you won't get the results you're expecting. This can be done at the commandline with:
mount --bind /src/www/test.com /home/user/test.com
If you want to add it to the
/etc/fstab file, it would look like:
/srv/www/test.com /home/user/test.com bind defaults,bind 0 0
However, I'd carefully consider why a symbolic link isn't sufficient before dealing with