oeqa/selftest/base: backup and restore local configuration files
Selftests' cleanup method during test setup is not capable of
restoring local configuration files that remain modified after
aborting a test through a keyboard interruption.
This change creates backups for local.conf and bblayers.conf at
test setup, restore them when found, and deletes them at cleanup.
[YOCTO #9390]
Signed-off-by: Jose Lamego <jose.a.lamego@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
