Add ability to append to rules/macros

Add the ability to append to rules/macros, like we already do with
lists. For rules/macros, if the object has an append: true key, the
condition value is appended to the condition of an existing rule/macro
with the same name.

Like lists, it's an error to specify append: true without there being an
existing rule/macro.

Also add tests that test the same kind of things we did for lists:
 - That append: true really does append
 - That append: false overwrites the rule/macro
 - That it's an error to append with a prior rule/macro existing.

test/falco_tests.yaml

@@ -599,3 +599,44 @@ trace_files: !mux
     rules_file:
       - rules/list_append_false.yaml
     trace_file: trace_files/cat_write.scap
+
+  macro_append_failure:
+    exit_status: 1
+    stderr_contains: "Macro my_macro has 'append' key but no macro by that name already exists. Exiting"
+    rules_file:
+      - rules/macro_append_failure.yaml
+    trace_file: trace_files/cat_write.scap
+
+  macro_append:
+    detect: True
+    detect_level: WARNING
+    rules_file:
+      - rules/macro_append.yaml
+    trace_file: trace_files/cat_write.scap
+
+  macro_append_false:
+    detect: False
+    rules_file:
+      - rules/macro_append_false.yaml
+    trace_file: trace_files/cat_write.scap
+
+  rule_append_failure:
+    exit_status: 1
+    stderr_contains: "Rule my_rule has 'append' key but no rule by that name already exists. Exiting"
+    rules_file:
+      - rules/rule_append_failure.yaml
+    trace_file: trace_files/cat_write.scap
+
+  rule_append:
+    detect: True
+    detect_level: WARNING
+    rules_file:
+      - rules/rule_append.yaml
+    trace_file: trace_files/cat_write.scap
+
+  rule_append_false:
+    detect: False
+    rules_file:
+      - rules/rule_append_false.yaml
+    trace_file: trace_files/cat_write.scap
+