sanitycheck: Execute binary in output directory

Execute the test binary from the output directory instead of directory
where sanitycheck was started.

This will ensure that any artifact created with a relative path by the
test binary will be placed in the output directory instead of creating
the artifact in the directory where sanitycheck was executed and prevent
any possible conflicts.

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>