Introduce pytest verify_keys and check_keytimes

Introduce pytest verify_keys and check_keytimes

This commit introduces replacements for the 'check_keys' and
'check_keytimes' from the shell test library. 'check_keys' is renamed
to 'verify_keys' because it does not assert.

For that, we introduce more functions for the class Key. The
'match_properties' function is used in 'verify_keys' to see if a set of
KeyProperties match the Key. This speficially ignores timing metadata.
The function resembles what is in 'kasp.sh:check_key()'.

The 'match_timingmetadata' function is used in 'check_keytimes' to see
if the timing metadata of a set of KeyProperties match the Key. The
values are checked in all three key files (except if the private key is
not available (set with properties["private"]), or if it is a legacy key
(set with properties["legacy"]).

An additional check function is added, to check if the key relationships
are set correctly. It follows a similar pattern as 'check_keytimes'. If
"Predecessor" and/or "Successor" are expected to be set in the state
file, this function checks so, and also verifies that they are not set
if they should not be.