Enable warnings-as-errors everywhere in CMake

[steven-johnson]

In CMake, it looks like we only set -Werror and friends for Halide itself. We should also do so for tests, apps, etc.

(And in general have the same set of warnings/errors for all of them too.)

[alexreinking]

Since CMake 3.24 (we are now on 3.28), we can set -DCMAKE_COMPILE_WARNING_AS_ERROR=ON in the buildbots to achieve this.