Fix unnecessary dependency scanning for test build and test rom names

[ravepossum]

Description

This fixes unnecessary extra dependency scanning currently being performed by the Makefile that's slowing down builds quite a bit. It occurs on every build, not just test ones.

The changes are based on the ones in this pret PR from pfero.

It also fixes the test ROM names erroneously including "agbcc" (which was causing them to not get caught by make clean).

Issue(s) that this PR fixes

• Builds being slow after the recent makefile merged (notable by multiple people in discord)

Discord contact info

ravepossum

[AsparagusEduardo]

Someone else should confirm that the Makefile has no issues, but I can at least confirm a big increase in speed (using 16 threads).

Before

real    1m27.489s
user    6m8.390s
sys     0m45.279s

After

real    0m53.457s
user    4m46.168s
sys     0m34.685s

[hedara90]

Everything is much faster.
sha1sum of built rom doesn't change.
sha1sum of built tests doesn't change.