Remove global test thread locking

With the recent changes in bootutil, now there is no need to lock the
test threads to run sequentially, so the global lock was removed. The
locking now happens to access resources such as flash, flash params,
simulator context, etc on a per thread basis. Some of the global
variables that were used by the simulator itself (FFI) were made into a
context struct passed in to invoke_go.

Signed-off-by: Fabio Utzig <utzig@apache.org>