selftest: Optimize get_bb_var use

selftest: Optimize get_bb_var use

get_bb_var calls bitbake every time it is used and every call
would take about 7 seconds. There are tests that calls get_bb_var
several times when they can use get_bb_vars. Also there are tests
that calls it to fetch the same variable over and over again.

This will optimize the use of get_bb_var and get_bb_vars for a
little speed up in the tests.

[YOCTO #11037]

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>