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/csr.hpp b/csr.hpp
index c6b4221..2d82739 100644
--- a/csr.hpp
+++ b/csr.hpp
@@ -1,9 +1,7 @@
 #pragma once
 #include <xyz/openbmc_project/Certs/CSR/server.hpp>
 
-namespace phosphor
-{
-namespace certs
+namespace phosphor::certs
 {
 using CSRRead = sdbusplus::xyz::openbmc_project::Certs::server::CSR;
 using CSRIface = sdbusplus::server::object::object<CSRRead>;
@@ -54,5 +52,4 @@
     /** @brief Status of GenerateCSR request */
     Status csrStatus;
 };
-} // namespace certs
-} // namespace phosphor
+} // namespace phosphor::certs