Silence Clang warnings about unused variables. NFC.

    warning: unused variable 'nullopt' [-Wunused-const-variable]
     2284 | const nullopt_t nullopt{};
          |                 ^~~~~~~

    warning: unused variable 'u' [-Wunused-variable]
      610 |     nonstd::unexpected<int> u{7};
          |                             ^

test/expected.t.cpp

@@ -607,7 +607,7 @@ CASE( "make_unexpected_from_current_exception(): Allows to create an unexpected_
 CASE( "unexpected: C++17 and later provide unexpected_type as unexpected" )
 {
 #if nsel_CPP17_OR_GREATER || nsel_COMPILER_MSVC_VERSION > 141
-    nonstd::unexpected<int> u{7};
+    [[maybe_unused]] nonstd::unexpected<int> u{7};
 #else
     EXPECT( !!"unexpected is not available (no C++17)." );
 #endif
@@ -2281,7 +2281,7 @@ CASE( "invoke" )
 
 struct nullopt_t{};
 
-const nullopt_t nullopt{};
+[[maybe_unused]] const nullopt_t nullopt{};
 
 /// optional expressed in expected