Deal with the issue identified by valgrind

[splasky]

• Fix memory leak detected by valgrind

This commit fixed the memory leak problem inside the tests directory.

• 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.)
  No.
• Does this PR change the the list of algorithms available -- either adding, removing, or renaming? (If so, PRs in OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also be required by the time this is merged.)
  No.

[baentsch]

Thanks for this catch and your contribution!