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