buildRustCrate: fixup integration test mod name

Fixes #204051. I have tried this on the reproducer stated in the ticket.

```
[nix-develop]$ $(nix-build -I nixpkgs=/home/shana/programming/nixpkgs --no-out-link)/tests/foo

running 1 test
test check_module_name ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```

pkgs/build-support/rust/build-rust-crate/lib.sh

···
 build_bin_test_file() {
     local file="$1"
     local derived_crate_name="${file//\//_}"
+    # Make sure to strip the top level `tests` directory: see #204051. Note that
+    # a forward slash has now become an underscore due to the substitution
+    # above.
+    derived_crate_name=${derived_crate_name#"tests_"}
     derived_crate_name="${derived_crate_name%.rs}"
     build_bin_test "$derived_crate_name" "$file"
 }