sanity: Don't make assumptions about cwd

When using the recently fixed out of build directory bitbake invocations, I was
puzzled why bitbake seemed to be pausing. The reason was due to running the sanity
tests each and every time. This was due to current working directory assumptions
within the sanity test code. Fix this to use TOPDIR.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>