Fix off-by-one error with lines from request.node.warn

The line numbers in `node.location` seem to be zero-based?!

testing/test_config.py

@@ -525,13 +525,14 @@ class TestWarning:
         reprec = testdir.inline_run()
         reprec.assertoutcome(passed=1)
 
-    def test_warn_on_test_item_from_request(self, testdir):
+    def test_warn_on_test_item_from_request(self, testdir, request):
         testdir.makepyfile("""
             import pytest
 
             @pytest.fixture
             def fix(request):
                 request.node.warn("T1", "hello")
+
             def test_hello(fix):
                 pass
         """)
@@ -542,7 +543,7 @@ class TestWarning:
         result = testdir.runpytest("-rw")
         result.stdout.fnmatch_lines("""
             ===*pytest-warning summary*===
-            *WT1*test_warn_on_test_item*:5*hello*
+            *WT1*test_warn_on_test_item*:7 hello*
         """)
 
 class TestRootdir: