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.cpp b/bmc-vmi-ca/ca_certs_manager.cpp
index 3b11a53..0f4e908 100644
--- a/bmc-vmi-ca/ca_certs_manager.cpp
+++ b/bmc-vmi-ca/ca_certs_manager.cpp
@@ -9,9 +9,7 @@
 #include <phosphor-logging/log.hpp>
 #include <xyz/openbmc_project/Common/error.hpp>
 
-namespace ca
-{
-namespace cert
+namespace ca::cert
 {
 static constexpr auto maxCertSize = 4096;
 namespace fs = std::filesystem;
@@ -66,5 +64,4 @@
     }
 }
 
-} // namespace cert
-} // namespace ca
+} // namespace ca::cert