Add tools/mod to module_dirs

As module_dirs also contains the go.mod file. We should add it into the
module_dirs variable, so when executing ./scripts/fix.sh, the proper
checks and fixes can be applied.

Discovered when working on #18575

The directories are checked against the following:
[Command]
find . -type f -name go.mod -exec dirname {} \;
[Output]
./etcdutl
.
./tools/testgrid-analysis
./tools/rw-heatmaps
./tools/mod
./etcdctl
./tests
./server
./api
./client/internal/v2
./client/v3
./client/pkg
./pkg

Signed-off-by: Chun-Hung Tseng <[email protected]>

scripts/test_lib.sh

@@ -166,7 +166,7 @@ function run_for_module {
 }
 
 function module_dirs() {
-  echo "api pkg client/pkg client/internal/v2 client/v3 server etcdutl etcdctl tests tools/rw-heatmaps tools/testgrid-analysis ."
+  echo "api pkg client/pkg client/internal/v2 client/v3 server etcdutl etcdctl tests tools/mod tools/rw-heatmaps tools/testgrid-analysis ."
 }
 
 # maybe_run [cmd...] runs given command depending on the DRY_RUN flag.