Fix filesystem::path::lexically_normal algorithm

Fix filesystem::path::lexically_normal algorithm

	* src/filesystem/std-path.cc (path::lexically_normal): Add missing
	step to algorithm, for removing dot-dot elements after root-directory.
	* testsuite/27_io/filesystem/operations/canonical.cc: Use
	compare_paths for more exhaustive checks.
	* testsuite/27_io/filesystem/operations/proximate.cc: Likewise.
	* testsuite/27_io/filesystem/path/append/path.cc: Likewise.
	* testsuite/27_io/filesystem/path/concat/path.cc: Likewise.
	* testsuite/27_io/filesystem/path/concat/strings.cc: Fix comment.
	* testsuite/27_io/filesystem/path/construct/locale.cc: Likewise.
	* testsuite/27_io/filesystem/path/decompose/root_directory.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/generation/normal.cc: Use
	compare_paths for more exhaustive checks. Add extra testcases.
	* testsuite/27_io/filesystem/path/generation/proximate.cc: Use
	compare_paths for more exhaustive checks.
	* testsuite/27_io/filesystem/path/generation/relative.cc: Likewise.
	* testsuite/27_io/filesystem/path/generic/generic_string.cc: Remove
	unused header.
	* testsuite/27_io/filesystem/path/modifiers/make_preferred.cc: Fix
	comment.
	* testsuite/27_io/filesystem/path/modifiers/remove_filename.cc: Use
	compare_paths for more exhaustive checks.
	* testsuite/27_io/filesystem/path/modifiers/replace_extension.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/modifiers/replace_filename.cc:
	Likewise.
	* testsuite/util/testsuite_fs.h (compare_paths): Also compare native
	strings.

From-SVN: r254075