test: Add basic kem fuzz testing

[nathaniel-brough]

Adds basic fuzz harness for kem algorithms

This is part of work towards: #1215

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[nathaniel-brough]

Looks like the CI is failing due to something unrelated. The disk on the Travis CI seems to be full when fetching build dependencies.

I'll have a closer look over the next week. Unless this is a known issue?

[dstebila]

Looks like the CI failure is resolved.

I think the fuzz test itself looks fine. Do we need to code to trigger this to actually run in the CI?