oe-selftest: force some values in local.conf for _sstatetests_noauto.py

oe-selftest: force some values in local.conf for _sstatetests_noauto.py

1) Some values should have default values in order for tests like sstate relocation to be relevant
   - SSTATE_DIR needs to have default value
   - SSTATE_MIRRORS need to be unset (we need the sstate files to be created)
   - TMPDIR needs to be in default location

2) Added conf/auto.conf to list of ported files to secondary build directories

(From OE-Core rev: 3b98de79e14c44e70fd72ed94493a674444bdba2)

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>