Add more tests for unionized sections + fix bugs

Implementing those tests found a few bugs... oops

test/link/section-union/no-room.asm

@@ -0,0 +1,12 @@
+; NB: the error is that there is no room to place the unionized section in,
+;     so RGBASM can't possibly error out.
+IF !DEF(SECOND)
+	SECTION "bloat", WRAMX[$d000], BANK[2]
+		ds $fe0
+
+	SECTION UNION "test", WRAMX, BANK[2]
+		db
+ELSE
+	SECTION UNION "test", WRAMX, ALIGN[6]
+		db
+ENDC