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

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.