libstdc++: Skip tests that fail for the versioned namespace

Most tests for the contents of header synopses need to be supressed for
the versioned namespace build, because redeclaring the entities in std
fails when they were originally declared in std::__8.

I added these tests recently without the suppression, so they fail.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/expected/synopsis.cc: Skip for versioned
	namespace.
	* testsuite/27_io/headers/iosfwd/synopsis.cc: Likewise.

(cherry picked from commit 1815462a6e)

libstdc++-v3/testsuite/20_util/expected/synopsis.cc

@@ -1,5 +1,6 @@
 // { dg-options "-std=gnu++23" }
 // { dg-do compile { target c++23 } }
+// { dg-require-normal-namespace "" }
 
 #include <expected>
 

libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc

@@ -1,4 +1,5 @@
 // { dg-do compile }
+// { dg-require-normal-namespace "" }
 
 #include <iosfwd>