Below are some test chambers that will be in Portal_Z. Note that the test chambers will not run properly unless you have downloaded the necessary sounds and textures.
To play a test chamber: Save the BSP files to one of these locations:
Windows 32-bit: C:\Program Files\Steam\steamapps\[yourSteamUserName]\portal\portal\maps
Windows 64-bit: C:\Program Files (x86)\Steam\steamapps\[yourSteamUserName]\portal\portal\maps
Mac OS X: ~/Library/Application Support/Steam/steamapps/[yourSteamUserName]/portal/portal/maps
To run a specific map, launch Portal, open the console (you can enable the console in Options > Keyboard > Advanced) and type in "map z_chamber_#" where "#" is the number in the file name of the chamber you downloaded.