Add some basic regression tests.

Most cause crashes. Only macro-@ works correctly; the others need to
be fixed.

test/asm/bank-noexist.asm

@@ -0,0 +1,2 @@
+SECTION "sec", ROM0
+	db BANK(noexist) 

test/asm/bank-noexist.out

@@ -0,0 +1,2 @@
+ERROR:	bank-noexist.asm(2) :
+	'noexist' not defined

test/asm/divzero-section-bank.asm

@@ -0,0 +1 @@
+SECTION "sec", ROMX[$/0]

test/asm/divzero-section-bank.out

@@ -0,0 +1,3 @@
+ERROR:	foo.asm(1) :
+	syntax error
+rgbasm: Assembly aborted in pass 1 (1 errors)!

test/asm/macro-@.asm

@@ -0,0 +1 @@
+foo @bar

test/asm/macro-@.out

@@ -0,0 +1,2 @@
+ERROR:	macro-@.asm(1) -> @(-1) :
+	Macro '@' not defined

test/asm/test.sh

@@ -0,0 +1,7 @@
+fname=$(mktemp)
+
+for i in *.asm; do
+	../../rgbasm $i >$fname 2>&1
+#	diff -u $fname $(basename $i .asm).out
+	diff -u $fname ${i%.asm}.out
+done

test/asm/undefined-dot.asm

@@ -0,0 +1,3 @@
+SECTION "sec", ROM0
+foo:
+	add sp, .

test/asm/undefined-dot.out

@@ -0,0 +1,2 @@
+ERROR:	undefined-dot.asm(3) :
+	'.' not defined