libstdc++: Fix -Wrange-loop-construct warnings in filesystem tests

libstdc++: Fix -Wrange-loop-construct warnings in filesystem tests

Many tests do `for (const path& p : test_paths)` where test_paths is an
array of strings. To avoid -Wrange-loop-construct warnings the loop
variable should be an object, not a reference bound to a temporary.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/filesystem/operations/absolute.cc: Avoid
	-Wrange-loop-construct warning.
	* testsuite/27_io/filesystem/path/append/source.cc: Likewise.
	* testsuite/27_io/filesystem/path/assign/copy.cc: Likewise.
	* testsuite/27_io/filesystem/path/compare/path.cc: Likewise.
	* testsuite/27_io/filesystem/path/construct/copy.cc: Likewise.
	* testsuite/27_io/filesystem/path/decompose/extension.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/decompose/filename.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/decompose/parent_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/decompose/relative_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/decompose/root_directory.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/decompose/root_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/itr/traversal.cc: Likewise.
	* testsuite/27_io/filesystem/path/modifiers/remove_filename.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/modifiers/replace_extension.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/modifiers/replace_filename.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/nonmember/append.cc: Likewise.
	* testsuite/27_io/filesystem/path/nonmember/cmp.cc: Likewise.
	* testsuite/27_io/filesystem/path/nonmember/cmp_c++20.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/nonmember/hash_value.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_extension.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_filename.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_parent_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_relative_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_root_directory.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_root_name.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_root_path.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/query/has_stem.cc: Likewise.
	* testsuite/27_io/filesystem/path/query/is_relative.cc: Likewise.
	* testsuite/experimental/filesystem/operations/absolute.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/assign/copy.cc: Likewise.
	* testsuite/experimental/filesystem/path/compare/path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/construct/copy.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/extension.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/filename.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/parent_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/relative_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/root_directory.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/decompose/root_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/itr/traversal.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/modifiers/remove_filename.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/modifiers/replace_extension.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/modifiers/replace_filename.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/nonmember/hash_value.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_extension.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_filename.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_parent_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_relative_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_root_directory.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_root_name.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_root_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/has_stem.cc:
	Likewise.
	* testsuite/experimental/filesystem/path/query/is_relative.cc:
	Likewise.