libstdc++: Fix incorrect test for std::error_code comparisons

libstdc++: Fix incorrect test for std::error_code comparisons

The tests for std::error_code comparisons assumed that a default
constructed object uses std::generic_category(). That's true for a
default constructed std::error_condition, but not std::error_code.

Fix the three-way comparisons to correctly depend on the result of
comparing the categories, and add another test for comparing two objects
with the same category and different values.

libstdc++-v3/ChangeLog:

	* testsuite/19_diagnostics/error_code/operators/not_equal.cc:
	Add comparison with same category and different values.
	* testsuite/19_diagnostics/error_code/operators/less.cc:
	Likewise. Fix comparison involving different categories.
	* testsuite/19_diagnostics/error_code/operators/three_way.cc:
	Likewise.
	* testsuite/19_diagnostics/error_condition/operators/less.cc:
	Add comment.
	* testsuite/19_diagnostics/error_condition/operators/three_way.cc:
	Likewise.