Inside the UMD of the PSP version is text indicating that there were at one time 11 test rooms in Sonic Rivals. All but Test Room 2 were removed from the final release. The code below can be used with CWCheat to access the room. The room itself isn't playable since there is no starting point.
|0x201533E8.dword = 0x74736574 + 0x201533EC.dword = 0x6F6F725F + 0x201533F0.dword = 0x0000326D||Test Level 2|