Can I install unsigned Windows drivers to work without test mode or disabling driver signing and without buying a code signing certificate?

Can I sign drivers with a "self-signed" certificate, to work just with this PC, of course, and not have to use DDISABLE_INTEGRITY_CHECKS, TESTSIGNING or Test Mode?

Many modded drivers for legacy devices (e.g., ppjoy) are unsigned don't work in x64 Windows Vista, Windows 7 or Windows 8.

If this is possible, how?

No, it's an either or situation. You can either have unsigned drivers + test mode on, or signed only drivers and test mode off.

If you don't like the watermark, there are ways to remove it, IIRC correctly the test mode watermark is generated by strings in the .mui files shell32.dll.mui and basebrd.dll.mui. That's basically what apps like DSEO (driver signing enforcement overrider) do.