ci: only run tests for libunbound

In particular, the auto-imported docs in libunbound-sys aren't
fully rustdoc conforming and would otherwise cause tests to fail.

This is a bit of a cruddy work around, but it's good enough for me
for now.

.github/workflows/rust.yml

@@ -19,4 +19,4 @@ jobs:
     - name: Build
       run: cargo build
     - name: Run tests
-      run: cargo test
+      run: cargo test -p libunbound