libstdc++: Fix accidental duplicate test [PR91456]

libstdc++: Fix accidental duplicate test [PR91456]

It looks like I committed the testcase for std::function twice, instead
of one for std::function and one for std::is_invocable_r. This replaces
the is_invocable_r one with the example from the PR.

libstdc++-v3/ChangeLog:

	PR libstdc++/91456
	* testsuite/20_util/function/91456.cc: Add comment with PR
	number.
	* testsuite/20_util/is_invocable/91456.cc: Likewise. Replace
	std::function checks with std::is_invocable_r checks.