GB: Test 16-bit reads from $3FFF

[pinobatch]

Lycoder, developer of GB emulator Geebly, suspected that some emulators might have trouble with 16-bit reads across the MBC breakpoint. The case we identified was a stack copy from $3FFF:

ld sp, $3FFF
pop de

calc84maniac, developer of TI-Boy CE, reminded the channel to test with multiple MBC banks. I generalized to these:

Now:

• [ ] Test $7FFF to $8000 (ROM to VRAM)
• [ ] Test $DFFF to $E000 (WRAM to echo RAM)

After the ROM exceeds 32 KiB and MBC is added:

• [ ] Test $3FFF to $4000 with both bank 1 and other than bank 1
• [ ] Test $7FFF to $8000 with both bank 1 and other than bank 1

After Game Boy Color is added:

• [ ] Test $7FFF to $8000 with both VBK settings
• [ ] Test $CFFF to $D000 with multiple SVBK settings
• [ ] Test $DFFF to $E000 with multiple SVBK settings

As usual, if everything passes including "usual culprits"-tier emulators (VBA, Goomba, no$gmb, KiGB, and rew.), don't make it a coin. It may be wise to make an exerciser first if possible.