Make self-tests run after install by a typical user

[noloader]

Currently our self tests only work when run from the build directory. The behavior can be changed such that they run from the install directory, but then the self tests don't run from the build directory.

To accommodate both cases we need to check the install location and the build directory. However, Debian err'd on the side of security and asked us to avoid searching. We honored Debian's request because it was a sound decision.

Revisiting the strategy I think we want the common case to be, the self tests "just work" under reasonable use cases. That is, they work when run from the build directory, and they work when run from the install directory. This helps regular users the most.

We can add a define to disable searching for test vectors for Debian.

[noloader]

Commit 1c88fd6f5956 took care of this issue. Users can run the self tests from the current directory or the install directory and things should "just work".

A new define was added, CRYPTOPP_DISABLE_DATA_DIR_SEARCH, to preserve the former behavior. Folks like Debian can activate it if they like.