oeqa/sstatetests: Fix NATIVELSBSTRING handling

oeqa/sstatetests: Fix NATIVELSBSTRING handling

The NATIVELSBSTRING variable changes value once a BuildStarted event occurs in a build
directory. This meant running some of the tests directly in a fresh build directory
would fail but they'd pass when run as a group of tests. This is clearly suboptimal.

Move the NATIVELSBSTRING handling to a location where the value is consistent
and a comment about the interesting behaviour of the variable so it hopefully doesn't
catch out others in future.

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