Improve testing bracketed symbols

Also test EQU and _RS constants, as well as that EQUS errors out,
and that labels don't work.

test/asm/bracketed-symbols.asm

@@ -4,3 +4,17 @@ PRINTT "{x:X}\n"
 PRINTT "{X:X}\n"
 PRINTT "{d:X}\n"
 PRINTT "{b:X}\n"
+
+Y equ 1337
+PRINTT "{b:Y}\n"
+
+rsreset
+R rb 0
+PRINTT "{d:R}\n"
+
+S equs "You can't format me!"
+PRINTT "{X:S}\n"
+
+SECTION "Test", ROM0
+Label:
+PRINTT "{x:Label}\n"