Add `--only-resource-intensive-tests` command line option to only run the resource intensive annotated tests.

Previously on the nightly builds the `dtest-large` job was used as a replacement for the `dtest` job. In the pipelines today both dtest and dtest-large are executed, so dtest-large re-executing the non-intensive tests is a waste.

 patch by Mick Semb Wever; reviewed by Eduard Tudenhöfner for CASSANDRA-15729
diff --git a/conftest.py b/conftest.py
index 7cc3acc..34de30b 100644
--- a/conftest.py
+++ b/conftest.py
@@ -55,6 +55,8 @@
                      help="Control the number of data directories to create per instance")
     parser.addoption("--force-resource-intensive-tests", action="store_true", default=False,
                      help="Forces the execution of tests marked as resource_intensive")
+    parser.addoption("--only-resource-intensive-tests", action="store_true", default=False,
+                     help="Only run tests marked as resource_intensive")
     parser.addoption("--skip-resource-intensive-tests", action="store_true", default=False,
                      help="Skip all tests marked as resource_intensive")
     parser.addoption("--cassandra-dir", action="store", default=None,
@@ -476,6 +478,13 @@
                     deselect_test = True
                     logger.info("SKIP: Deselecting resource_intensive test %s due to insufficient system resources" % item.name)
 
+        if not item.get_closest_marker("resource_intensive") and not collect_only:
+            only_resource_intensive = config.getoption("--only-resource-intensive-tests")
+            if only_resource_intensive:
+                deselect_test = True
+                logger.info("SKIP: Deselecting non resource_intensive test %s as --only-resource-intensive-tests specified" % item.name)
+
+
         if item.get_closest_marker("no_vnodes"):
             if config.getoption("--use-vnodes"):
                 deselect_test = True