oeqa/selftest/wic: use self.td instead of get_bb_var to save on bitbake calls

oeqa/selftest/wic: use self.td instead of get_bb_var to save on bitbake calls

When a test case starts, self.td is populated with all the variables in
the data store.  Typically this can be used instead of get_bb_var(),
which saves a bitbake call per variable lookup.

The only catch is that in parallel runs the build directory is moved
after td is populated, so paths in the build directory are wrong: these
still need to be fetched in the test.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>