Use nested namespace

Nested namespace is introduced in C++ 17. This saves nearly 50 lines.
This change also puts tests into a nested namespace, which saves
unnecessary using directives.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I7c3e38588fd5c2cbd83ac13ee24327318e8c06a4
diff --git a/certs_manager.hpp b/certs_manager.hpp
index 122ce44..2be0759 100644
--- a/certs_manager.hpp
+++ b/certs_manager.hpp
@@ -11,9 +11,7 @@
 #include <xyz/openbmc_project/Certs/Install/server.hpp>
 #include <xyz/openbmc_project/Collection/DeleteAll/server.hpp>
 
-namespace phosphor
-{
-namespace certs
+namespace phosphor::certs
 {
 using Install = sdbusplus::xyz::openbmc_project::Certs::server::Install;
 using DeleteAll =
@@ -306,5 +304,4 @@
     /** @brief Certificate ID pool */
     uint64_t certIdCounter = 1;
 };
-} // namespace certs
-} // namespace phosphor
+} // namespace phosphor::certs