test: Add "addition" codegen test

test/simple_add.c0

@@ -0,0 +1,3 @@
+int main() {
+    return 1 + 1;
+}

test/simple_add.c0.expected

@@ -0,0 +1,15 @@
+.global main
+.global _main
+.text
+main:
+call _main
+# move the return value into the first argument for the syscall
+movq %rax, %rdi
+# move the exit syscall number into rax
+movq $0x3C, %rax
+syscall
+_main:
+# your generated code here
+  mov $1,%0
+  add %0,%0
+  ret

test/var_add.c0

@@ -0,0 +1,4 @@
+int main() {
+    int x = 1;
+    return x + 2;
+}

test/var_add.c0.expected

@@ -0,0 +1,16 @@
+.global main
+.global _main
+.text
+main:
+call _main
+# move the return value into the first argument for the syscall
+movq %rax, %rdi
+# move the exit syscall number into rax
+movq $0x3C, %rax
+syscall
+_main:
+# your generated code here
+  mov $1,%0
+  mov $2,%1
+  add %0,%1
+  ret