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/bmc-vmi-ca/ca_certs_manager.hpp b/bmc-vmi-ca/ca_certs_manager.hpp
index a1db918..21088b6 100644
--- a/bmc-vmi-ca/ca_certs_manager.hpp
+++ b/bmc-vmi-ca/ca_certs_manager.hpp
@@ -8,9 +8,7 @@
 #include <sdbusplus/server/object.hpp>
 #include <sdeventplus/source/event.hpp>
 
-namespace ca
-{
-namespace cert
+namespace ca::cert
 {
 
 class CACertMgr;
@@ -74,5 +72,4 @@
     uint32_t lastEntryId;
 };
 
-} // namespace cert
-} // namespace ca
+} // namespace ca::cert