shpec should use proper 'temp' files

[hlangeveld]

Currently, the default tests for shpec in shpec_shpec.sh use hardcoded temporary files.
This can cause strange effects on systems where these filenames have been used before, whether by the same user or by others.

Example: /tmp/syntax_error_output

What we need is a couple of helper functions and hooks to create proper temporary files (using the external mktemp(1) on linux systems.

When we create tempfiles, we can add them to a ${_shpec_temp_files} list so we can remove them in end().

Wanders of pondering on storing filename with indent_level...

[nwmcsween]

mktemp isn't POSIX, see: https://github.com/nwmcsween/shrimp/blob/master/file.sh for a portable way of doing it.