test: properly tag anonymous namespaces

For tests that use anonymous namespaces, some tagged the close
of the namespace with 'namespace' while others used
'anonymous namespace'. It was suggested I should use
'anonymous namespace' in a recent PR review so make all of the
tests consistent with this.

PR-URL: https://github.com/nodejs/node/pull/18583


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>