libstdc++: Fix tests with non-const operator==

These tests fail in strict -std=c++20 mode but their equality ops don't
need to be non-const, it looks like an accident.

This fixes two FAILs with -std=c++20:
FAIL: 20_util/tuple/swap.cc (test for excess errors)
FAIL: 26_numerics/valarray/87641.cc (test for excess errors)

libstdc++-v3/ChangeLog:

	* testsuite/20_util/tuple/swap.cc (MoveOnly::operator==): Add
	const qualifier.
	* testsuite/26_numerics/valarray/87641.cc (X::operator==):
	Likewise.

libstdc++-v3/testsuite/20_util/tuple/swap.cc

@@ -38,7 +38,7 @@ struct MoveOnly
   MoveOnly(MoveOnly const&) = delete;
   MoveOnly& operator=(MoveOnly const&) = delete;
 
-  bool operator==(MoveOnly const& m)
+  bool operator==(MoveOnly const& m) const
   { return i == m.i; }
 
   void swap(MoveOnly& m)

libstdc++-v3/testsuite/26_numerics/valarray/87641.cc

@@ -39,7 +39,7 @@ struct X
   X() : val(1) { }
 
   X& operator+=(const X& x) { val += x.val; return *this; }
-  bool operator==(const X& x) { return val == x.val; }
+  bool operator==(const X& x) const { return val == x.val; }
 
   int val;
 };