unit-test: cppcheck: use temporary directory
unit-test: cppcheck: use temporary directory

The previous code would create a directory named `cppcheck-temp`
in the root of the repository under test as the cppcheck cache but
never cleaned it up.  This left stray directories around when using
the unit test locally.

Switch the mechanism to use tempfile.TemporaryDirectory so that
the directory is automatically cleaned up when the testing is
complete.

Tested: Ran unit-test locally and observed cppcheck-temp directory
no longer remains[1].

1. https://github.com/williamspatrick/dotfiles/blob/840250f0c56b299a8de47cc2ea7c558a14eeb517/env/30_linux/lfopenbmc.zsh#L76

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: Idde8feb28bef663d156c5d1d1f21a3d8c629c72d

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts