test(diff_snapshot_works_after_error): do not rely on log

Since we deliberately create a condition of no space, it is not
guaranteed that Firecracker is able to produce complete log messages.
Use the API error message instead.

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

tests/integration_tests/functional/test_snapshot_not_losing_dirty_pages.py

@@ -51,13 +51,8 @@ def test_diff_snapshot_works_after_error(
 
     subprocess.check_call(f"fallocate -l {target_size} {fill}", shell=True)
 
-    try:
+    with pytest.raises(RuntimeError, match="No space left on device"):
         uvm.snapshot_diff()
-    except RuntimeError:
-        msg = "No space left on device"
-        uvm.check_log_message(msg)
-    else:
-        assert False, "This should fail"
 
     fill.unlink()