Allow tests that require libolm to be opted out of

[kegsay]

So:

• A) we don't need to force people to install libolm when they don't run crypto tests
• B) we can import different versions of mautrix without causing version incompatibilities

[kegsay]

Given libolm is deprecated and we don't really have any tests which actually use it, and that we have complement-crypto for checking crypto, I'm inclined to remove the dep, to A) simplify the install process and B) not cause version conflicts - cc @sumnerevans