test/py: always use autostart on tpm2 selftests

commit 789ed278 ("test/py: replace 'tpm2 init, startup, selftest' sequences")
changed some of the tpm2 init sequences to 'tpm2 autostart' instead of
calling 'tpm init', 'tpm startup TPM2_SU_CLEAR', 'tpm2 self_test full'.

The autostart command calls the afforementioned sequence and on top of
that deals with the 'tpm2 init' return codes if the tpm is already
started. Since we initialize the tpm from various subsystems now,
replace the last remaining instances of 'tpm2 init' with 'tpm2
autostart'.  Since the latter calls 'tpm2 init' anyway we will still be
implicitly testing the validity of that command

It's worth noting that since 'tpm2 autostart' performs the startup and
self tests sequences of the tpm we could drop
'test_tpm2_sandbox_self_test_full' and 'test_tpm2_startup, but let's
keep the since they test tpm commands and options

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>