Make std.file.isFile, isSymlink and their unittests safe

`std.file.isFile`, `isSymlink`, a unittest for `isFile` and a unittest for `isSymlink` on Windows systems do not contain any unsafe operations.
On Posix systems, a unittest for `isSymlink` contains the use of an unsafe function `core.sys.posix.unistd.symlink` but we can verify that it can be trusted.

In the unittest, it is used in the following form: `trustedSymlink(system_directory, symfile)` and `trustedSymlink(system_file, symfile)`.
Both `symfile` are defined to be null-terminated. But I missed `system_directory` and `system_file` are not null-terminated.
I will fix it soon.