[basm] Add more test to directive bank

[rgiot]

In the following code

	BANK &C4
	org &4000
start_c4

BITMAPS	incbin "files/BITMAPS3.SCR"	
stop_c4
	print "start_c4: ", {hex}start_c4
	print "stop_c4: ", {hex}stop_c4

start_c4 is equal to 0x4000, while stop_c4 is equal to 0x0000 instead of 0x8000 (I think it should be the case, at least).

it would be useful to check what is the behavior in other assemblers such as rasm and winape.

And fix this bug (or remove this feature ;))

[rgiot]

yep, the bank directive does not seem to work pretty well when saving content spread other banks because addresse computations seem wrong
bank &C6
c6_start
read "music.asm"
c6_stop
; c6_length equ $-c6_start ; XXX this seems buggy
c6_length equ c6_stop - c6_length