tests: Extend tpm2 test with key context save / load across updates

This patch adds a test case that performs a key context load on a
key context that was previously generated and is expected to be usable
after an upgrade of the TPM 2 while for example the a VM was suspend
and is resumed after the upgrade of libtpms.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>