test: Add dbus handler unit tests

Created unit tests for all of the object mapper dbus handlers.

Refactor the handlers to handler.cpp to allow unit testing.
Also updated to use std::set_intersection.

Change-Id: Ib87ab81f949d27f7dab55899d3e8d380e43bda04
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/src/test/meson.build b/src/test/meson.build
index 07ef230..c377a64 100644
--- a/src/test/meson.build
+++ b/src/test/meson.build
@@ -1,5 +1,6 @@
 processing_cpp_dep = declare_dependency(sources: '../processing.cpp')
 associations_cpp_dep = declare_dependency(sources: '../associations.cpp')
+handler_cpp_dep = declare_dependency(sources: '../handler.cpp')
 
 tests = [
   [ 'well_known', [ associations_cpp_dep, processing_cpp_dep ]],
@@ -7,6 +8,7 @@
   [ 'associations', [ associations_cpp_dep ]],
   [ 'name_change', [ associations_cpp_dep, processing_cpp_dep ]],
   [ 'interfaces_added', [ associations_cpp_dep, processing_cpp_dep ]],
+  [ 'handler', [ handler_cpp_dep, sdbusplus, phosphor_dbus_interfaces ]],
 ]
 
 foreach t : tests