OpenSSL::Test: Fix directory calculations in __cwd()

We recalculate the location of the directories we keep track of.
However, we did so after having moved to the new directory already, so
the data we did the calculations from were possibly not quite correct.

This change moves the calculations to happen before moving to the new
directory.

This issue is sporadic, and possibly dependent on the platform.

Reviewed-by: Matt Caswell <matt@openssl.org>