bbconfigbuild/configfragments.py: set BBPATH in standalone datastore

bbconfigbuild/configfragments.py: set BBPATH in standalone datastore

Otherwise fragments with 'require' statements will not parse,
as the parser will not be able to find files referred to by
those statements.

Add such a statement to the test fragment so that the scenario
is tested.

[YOCTO #15707]

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>