enabling flexible build dir name in testing

[baentsch]

So far, all test executions were contingent upon the availability of the (hard-coded) build directory "build". This proves cumbersome if using differently named build directories, e.g., for different build options or build targets (e.g., liboqs binaries for openssh static build, openssl shared build, etc.).

This PR does away with this limitation, using the cmake variable CMAKE_BINARY_DIR instead.

• 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 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.)