Add reboot_test

This test spawns several services backed by /system/bin/yes executable,
and then stops them either while SIGTERM or SIGKILL.

Ideally we want to unit test more of reboot logic, but that requires a
bigger refactoring.

Test: atest CtsInitTestCases
Bug: 170315126
Bug: 174335499
Change-Id: Ife48b1636c6ca2d0aac73f4eb6f4737343a88e7a
diff --git a/init/Android.bp b/init/Android.bp
index 19ba21b..1efbdc0 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -261,6 +261,7 @@
         "persistent_properties_test.cpp",
         "property_service_test.cpp",
         "property_type_test.cpp",
+        "reboot_test.cpp",
         "rlimit_parser_test.cpp",
         "service_test.cpp",
         "subcontext_test.cpp",