test/py: tpm2: switch from 'tpm' to 'tpm2' command

While using the 'tpm' command should work on most cases, this test suite
only works with TPMv2 and since the work to make both versions build at
the same time, we might end up having both 'tpm' (TPMv1) and 'tpm2'
(TPMv2) commands available at the same time. Ensure this test suite
always use the right one.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>