Fix integration test file locking: close ifstream before fs::remove

On Windows, fs::remove throws when the file is still open by an ifstream
in the same process. Close the ifstream explicitly before removing the
annotations JSON temp files in the three affected tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: asklar

tests/integration_tests.cpp

@@ -390,6 +390,7 @@ TEST_F(NotepadFixture, AnnotationsJsonOutput) {
         EXPECT_GT(h, 0) << "Annotation " << a["id"] << " has non-positive height";
     }
 
+    f.close();
     fs::remove(annFile);
 }
 
@@ -428,6 +429,7 @@ TEST_F(NotepadFixture, AnnotationsMatchTreeElements) {
             << "Annotated element " << id << " not found in tree";
     }
 
+    f.close();
     fs::remove(annFile);
 }
 #endif
@@ -708,6 +710,7 @@ TEST_F(KnownWindowFixture, AnnotationsIncludeKnownControls) {
             << "Annotation " << a["id"] << " has non-positive height";
     }
 
+    f.close();
     fs::remove(annFile);
 }
 #endif