test(sandbox): use lastIndexOf to clean up watch flags

Address review feedback on #4104: when the test runner itself is started
with `node --watch`, the existing indexOf() cleanup would strip the
runner's flag. Since each watchFlag is appended via push(), lastIndexOf()
reliably targets our own entry without disturbing pre-existing values.

Author: mohanrajvenkatesan23-04

tests/child-pool.test.ts

@@ -142,9 +142,12 @@ function sandboxProcessTests(
         }
       } finally {
         // Remove only the flags we added so we don't disturb sibling tests
-        // that also mutate process.execArgv.
+        // that also mutate process.execArgv. We push() each flag onto the
+        // end, so lastIndexOf() targets our own entry — using indexOf()
+        // would remove a pre-existing --watch* if the runner itself was
+        // started with `node --watch`.
         for (const flag of watchFlags) {
-          const idx = process.execArgv.indexOf(flag);
+          const idx = process.execArgv.lastIndexOf(flag);
           if (idx !== -1) {
             process.execArgv.splice(idx, 1);
           }