added tests

Author: wassup05

test/system/test_filesystem.f90

@@ -1,6 +1,6 @@
 module test_filesystem
     use testdrive, only : new_unittest, unittest_type, error_type, check, skip_test
-    use stdlib_system, only: is_directory, delete_file
+    use stdlib_system, only: is_directory, delete_file, FS_ERROR, FS_ERROR_CODE
     use stdlib_error, only: state_type
 
     implicit none
@@ -13,6 +13,7 @@ subroutine collect_suite(testsuite)
         type(unittest_type), allocatable, intent(out) :: testsuite(:)
 
         testsuite = [ &
+            new_unittest("FS_ERROR", test_FS_ERROR), &
             new_unittest("fs_is_directory_dir", test_is_directory_dir), &
             new_unittest("fs_is_directory_file", test_is_directory_file), &
             new_unittest("fs_delete_non_existent", test_delete_file_non_existent), &
@@ -21,6 +22,24 @@ subroutine collect_suite(testsuite)
         ]
     end subroutine collect_suite
 
+    subroutine test_FS_ERROR(error)
+        type(error_type), allocatable, intent(out) :: error
+        type(state_type) :: s1, s2
+        character(:), allocatable :: msg
+
+        msg = "code - 10, Cannot create File temp.txt - File already exists"
+        s1 = FS_ERROR_CODE(10, "Cannot create File temp.txt -", "File already exists")
+
+        call check(error, s1%message == msg, "FS_ERROR: Could not construct message with code correctly")
+        if (allocated(error)) return
+
+        msg = "Cannot create File temp.txt - File already exists"
+        s2 = FS_ERROR("Cannot create File temp.txt -", "File already exists")
+
+        call check(error, s2%message == msg, "FS_ERROR: Could not construct message without code correctly")
+        if (allocated(error)) return
+    end subroutine test_FS_ERROR
+
     ! Test `is_directory` for a directory
     subroutine test_is_directory_dir(error)
         type(error_type), allocatable, intent(out) :: error