build: Align VS test folder with all other Generators

[res0nance]

This aligns the output folder for tests much like other platforms.

Fixes #1555

• 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 oqs-provider, OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[baentsch]

I like this approach (better): It's good that VS differentiates binaries between build types, but as ninja doesn't, this PR creates identical behaviour also eliminating the github env var dependency in the previous solution. It's certainly easier than replicating the VS behaviour in ninja builds...

[baentsch]

Merged: Thanks for the contribution!