tests: add filesystem api test

This commit verifies below FS APIs:
	fs_open()
	fs_close()
	fs_read()
	fs_write()
	fs_truncate()
	fs_seek()
	fs_tell()
	fs_sync()
	fs_unlink()
	fs_mkdir()
	fs_opendir()
	fs_closedir()
	fs_readdir()
	fs_stat()
	fs_statvfs()

Change-Id: I1a48b9e960b62a1c4986cc577f658199da7a28cd
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>