RFC: Build feature set shouldn't affect the total unit test list

Currently, the build system includes source files with unit tests depending on the actual build feature set, including options such as `ENABLE_WALLET`, `ENABLE_IPC` and `BUILD_GUI_TESTS`.

This leads to confusion when test results from different builds are aggregated together, for example, in https://my.cdash.org/index.php?project=bitcoin-core. It raises questions about why the total number of tests differs across builds.

The functional tests take a better approach by simply disabling tests for unavailable features.

I believe the unit tests should adopt the same approach.

In light of the ongoing modernization of the test framework (https://github.com/bitcoin/bitcoin/pull/33483 and https://github.com/bitcoin/bitcoin/pull/35713), I wonder whether those efforts are compatible with this idea.

Start by reading the linked test-framework modernization pull requests and compare how functional tests handle unavailable features with the current unit-test build behavior. Determine whether the proposed approach can keep the total unit-test list consistent across feature sets; done means the design is compatible with the ongoing modernization and the expected behavior is documented.

Written by the indexing model from the issue text.