tests: Switch to openapi-core 0.13.x

tests: Switch to openapi-core 0.13.x

We've done the necessary work here already so this is a relatively easy
switchover. However, we do have to work around an issue whereby the
first possible matching route is used rather than the best one [1]. In
addition, we have to install from master since there are fixes missing
from the latest release, 0.13.3. Hopefully both issues will be resolved
in a future release.

[1] https://github.com/p1c2u/openapi-core/issues/226

Signed-off-by: Stephen Finucane <stephen@that.guru>