lit: exclude mlir test case in benchgc folder

Author: WangJialei-A

test/lit.cfg.py

@@ -37,7 +37,7 @@
 # excludes: A list of directories to exclude from the testsuite. The 'Inputs'
 # subdirectories contain auxiliary inputs for various tests in their parent
 # directories.
-config.excludes = []
+config.excludes = ["benchgc"]
 
 # test_exec_root: The root path where tests should be run.
 config.test_exec_root = os.path.join(config.gc_obj_root, "test")