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/test/ca_certs_manager_test.cpp b/test/ca_certs_manager_test.cpp
index 17c8589..5bdad56 100644
--- a/test/ca_certs_manager_test.cpp
+++ b/test/ca_certs_manager_test.cpp
@@ -10,9 +10,12 @@
 
 #include <gtest/gtest.h>
 
+namespace ca::cert
+{
+namespace
+{
 using InvalidArgument =
     sdbusplus::xyz::openbmc_project::Common::Error::InvalidArgument;
-using namespace ca::cert;
 
 class MockCACertMgr : public CACertMgr
 {
@@ -123,3 +126,5 @@
     manager.erase(std::stoi(id));
     EXPECT_TRUE(manager.getNumOfEntries() == 0);
 }
+} // namespace
+} // namespace ca::cert