nixos/test-driver: separate the subtest log call

We do not use the generic "nested" function but introduce a separate
subtest log call. This will later allow us to track subtests and account
logs to specific subtests.

nixos/lib/test-driver/test_driver/driver.py

@@ -99,7 +99,7 @@ class Driver:
 
     def subtest(self, name: str) -> Iterator[None]:
         """Group logs under a given test name"""
-        with rootlog.nested("subtest: " + name):
+        with rootlog.subtest(name):
             try:
                 yield
                 return True

nixos/lib/test-driver/test_driver/logger.py

@@ -79,6 +79,11 @@ class Logger:
         except Empty:
             pass
 
+    @contextmanager
+    def subtest(self, name: str, attributes: Dict[str, str] = {}) -> Iterator[None]:
+        with self.nested("subtest: " + name, attributes):
+            yield
+
     @contextmanager
     def nested(self, message: str, attributes: Dict[str, str] = {}) -> Iterator[None]:
         self._eprint(