fix: give build_test a descriptive rule kind

[tomatotomata]

Summary

• rename the internal rule kind generated by build_test from _empty_test to skylib_build_test
• keep the public build_test macro and generated target names unchanged

This addresses #602 and makes bazel query 'kind(skylib_build_test, //...)' readable without changing the macro API.

Validation

• git diff --check
• existing //tests:build_test_test coverage remains the focused test target

Bazel 8.x is not installed in my Windows checkout, so I could not run the Bazel test locally. The commit includes a DCO sign-off.