selftest/fetch: Avoid occasional selftest failure from poor temp file name choice

The temp file name may contain "_" characters. Switch to a temporary directory
and a fixed filename to avoid this to avoid errors like:

bb.data_smart.ExpansionError: Failure expanding variable PN, expression was
${@bb.parse.vars_from_file(d.getVar('FILE', False),d)[0] or 'defaultpkgname'}
which triggered exception ParseError:
ParseError in /tmp/tmpd_f2__to.bb: Unable to generate default variables from
filename (too many underscores)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>