nixos/tests: Remove tests.boot.boot* prefixes

As @bobvanderlinden suggests in #13585:

"Looks like that cleans things up quite a bit! Just one aesthetics note,
the boot tests could now be renamed from boot.bootBiosCdrom to
boot.biosCdrom in nixos/tests/boot.nix:L33.

That makes them more consistent with the other tests."

Signed-off-by: aszlig <aszlig@redmoonstudios.org>