Enable running unit tests in GitLab CI

Do this for the library/local crate only.  Not the main binary nor
the examples can be tested for the x86_64-unknown-linux-gnu target.

.gitlab-ci.yml

@@ -26,6 +26,4 @@ rust-build:
   script:
   - rustc --version && cargo --version
   - cargo build --release --jobs 1
-  # FIXME: These should be run on the x86_64-unknown-linux-gnu target,
-  # does nothing otherwise
-  - env RUSTDOCFLAGS=-Cpanic=abort cargo test -- --test-threads 1
+  - cargo test --lib --target x86_64-unknown-linux-gnu  -- --test-threads 1