add tests for file module predicates

reproduce failing `directory_exists/1` with working directory passed as first argument
reproduce failing `delete_directory/1` with ./test directory passed as first argument
reproduce failing `file_size/1` with 2 bytes files passed as first argument
reproduce failing `file_exists/1` with existing file passed as first argument
reproduce failing `file_modification_time/1`, `file_access_time/1` and `file_creation_time/1` with existing file passed as first argument
reproduce failing `directory_files/2` with existing directory passed as first argument
reproduce failing `delete_file/1` with existing file passed as first argument
reproduce failing `make_directory/1` with non-existing directory passed as first argument
reproduce failing `make_directory_path/1` with non-existing path passed as first argument
reproduce failing `path_canonical/2` with non-canonical path passed as first argument
reproduce failing `rename_file/2` with existing file passed as first argument, target file as second arg
revised `directory_exists/1` test case
renamed test files, test directories
use `path_segments/2`, remove path separators
do not write file size to current output
revised `directory_files/2` test
revised `path_canonical/2` test
revised `file_exists/1` test
removed hardcoded expected size
extracted hardcoded directory argument
remove path prefix containing path separator
revised expected ls cmd exit code so that it is platform-agnostic

Signed-off-by: Thierry Marianne <thierry@marianne.io>

tests-pl/issue_file_copy.pl

@@ -0,0 +1,36 @@
+:- use_module(library(files)).
+:- use_module(library(iso_ext)).
+:- use_module(library(lists)).
+:- use_module(library(os), [setenv/2, getenv/2, shell/2]).
+
+check :-
+    act(Source, Destination),
+    ground(Source),
+    ground(Destination).
+
+act(Source, Destination) :-
+    getenv("SOURCE", Source),
+    getenv("DESTINATION", Destination),
+	append("ls ", Source, Cmd),
+	shell(Cmd, 0),
+	file_copy(Source, Destination),
+	append("ls ", Destination, Cmd),
+	\+ shell(Cmd, 0),
+	throw(existence_error(directory,Destination)).
+act(Source, Destination) :-
+    getenv("SOURCE", Source),
+    getenv("DESTINATION", Destination),
+	append("ls ", Destination, Cmd),
+	shell(Cmd, 0),
+    write(file_copied).
+
+main :-
+    call_cleanup(
+       (setenv("SOURCE", "file_copy_test_source"),
+        setenv("DESTINATION", "file_copy_test_destination"),
+        shell("touch file_copy_test_source", 0),
+        check),
+        shell("rm -f file_copy_test_source file_copy_test_destination", 0)
+    ).
+
+:- initialization(main).