init: create init_tests.cpp

Start a init_tests.cpp file for end-to-end tests that parse small init script
segments and verify that they act as expected.

The first tests ensure that the execution order of event triggers
happens appropriately.

Test: Boot bullhead, run unit tests

Change-Id: Ic446c02605ab796fd41e0596ce1fd381aee80ce0
diff --git a/init/builtins.cpp b/init/builtins.cpp
index 87d4e81..1d98ef1 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -838,7 +838,7 @@
     return e4crypt_do_init_user0();
 }
 
-BuiltinFunctionMap::Map& BuiltinFunctionMap::map() const {
+const BuiltinFunctionMap::Map& BuiltinFunctionMap::map() const {
     constexpr std::size_t kMax = std::numeric_limits<std::size_t>::max();
     // clang-format off
     static const Map builtin_functions = {