Calculate cumulative coverage by default.

tests/clickhouse-test

@@ -1322,6 +1322,7 @@ class TestCase:
         # We want to calculate per-test code coverage. That's why we reset it before each test.
         if (
             args.collect_per_test_coverage
+            and args.reset_coverage_before_every_test
             and BuildFlags.SANITIZE_COVERAGE in args.build_flags
         ):
             clickhouse_execute(
@@ -2843,6 +2844,12 @@ def parse_args():
         default=True,
         help="Create `system.coverage` table on the server and collect information about low-level code coverage on a per test basis there",
     )
+    parser.add_argument(
+        "--reset-coverage-before-every-test",
+        action="store_true",
+        default=False,
+        help="Collect isolated test coverage for every test instead of a cumulative. Useful only when tests are run sequentially.",
+    )
     parser.add_argument(
         "--report-logs-stats",
         action="store_true",