Split tests into their own `BUILD.bazel` files

Bazel module loading doesn't allow mixing public targets and targets that depend on dev dependencies (see bazelbuild/bazel#25187). The workaround is to move test targets into their own BUILD.bazel files.

Tests should move from //path/to/BUILD.bazel to //path/to/_tests/BUILD.bazel. Using _tests instead of tests makes it more discoverable (due to it appearing at the top lexically), and distinguishes it from namespace segments.