Remove rust_test example which doesn't build.

[cfredric]

I think this documentation has become out-of-date. If I try to build a rust_test target with no srcs attribute and no crate attribute, I get a build error at analysis time (Error in fail: No lib.rs or hello_lib_test.rs source file found. srcs).

If I understand correctly, the two variations of rust_test targets are:

• Unit tests: A rust_test target which specifies the crate attribute, has no srcs attribute, and may or may not have dev-dependenices in deps.
• Integration tests: A rust_test target which has a non-empty srcs attribute, includes a dependency on the SUT in deps (plus any additional dev-dependencies), and does not specify the crate attribute.

If that understanding is incorrect, please let me know!

[UebelAndre]

Thanks!

[UebelAndre]

If I understand correctly, the two variations of rust_test targets are:

* Unit tests: A `rust_test` target which specifies the `crate` attribute, has no `srcs` attribute, and may or may not have dev-dependenices in `deps`.

* Integration tests: A `rust_test` target which has a non-empty `srcs` attribute, includes a dependency on the SUT in `deps` (plus any additional dev-dependencies), and does not specify the `crate` attribute.

I don't think there's a hard distinction in Bazel between unit and integration tests but I do share the same mental picture here and would use this terminology (which I feel aligns with Cargo terminology where these are more explicitly defined).