Bring back --no-tests=warn when needed justfile

This was temporarily removed to verify that we no longer need to
set `NEXTEST_NO_TESTS` to `fail` explicitly in `ci.yml` to get that
failing behavior, now that it is implied starting in
`cargo-nextest` version 0.9.85.

It is now brought back, so that we do not get that error in the one
place where we know there are no test cases yet do tell `nextest`
to run tests.

justfile

@@ -186,7 +186,7 @@ unit-tests:
     cargo nextest run -p gix --no-default-features --features basic,extras,comfort,need-more-recent-msrv
     cargo nextest run -p gix --features async-network-client
     cargo nextest run -p gix --features blocking-network-client
-    cargo nextest run -p gitoxide-core --lib
+    cargo nextest run -p gitoxide-core --lib --no-tests=warn
 
 # These tests aren't run by default as they are flaky (even locally)
 unit-tests-flaky: