test/py: replace 'tpm2 init, startup, selftest' sequences

Instead of copy pasting the commands needed to start a TPM consisting
of:
- tpm init
- tpm startup TPM2_SU_CLEAR
- tpm2 self_test full
use the newly added 'autostart' which does the same thing and simplify
our python scripts

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>