More file based assert functions added

[peshay]

I changed assert_file_exists to assert_exists, since it's not just checking files but also directories. I added following new assert functions

• assert_file_exists/assert_file_not_exists - only test for a file
• assert_dir_exists/assert_dir_not_exists - only test for directory
• assert_link_exists/assert_link_not_exists - only test for symbolic link
• assert_block_exists/assert_block_not_exists - only test for block special files
• assert_character_exists/assert_character_not_exists - only test for character special files
• assert_socket_exists/assert_socket_not_exists - only test for socket
• assert_fifo_exists/assert_fifo_not_exists - only test for named pipes (FIFO)

[peshay]

Closing here, since not maintained for about 3 years. I'll start to maintain my own: https://github.com/peshay/bats-file