[ci] add an always tag for cond testing (#51662)

so that "lint" tag can be used for just lints, and will not be abused to
be used as "always"

Signed-off-by: Lonnie Liu <lonnie@anyscale.com>

Author: aslonnie

ci/pipeline/determine_tests_to_run.py

@@ -11,6 +11,7 @@
 
 _ALL_TAGS = set(
     """
+    always
     lint python cpp core_cpp java workflow accelerated_dag dashboard
     data serve ml tune train llm rllib rllib_gpu rllib_directly
     linux_wheels macos_wheels docker doc python_dependencies tools
@@ -189,6 +190,7 @@ def match_tags(self, changed_file: str) -> Tuple[Set[str], bool]:
 
     tags: Set[str] = set()
 
+    tags.add("always")
     tags.add("lint")
 
     def _emit(line: str):

ci/pipeline/test_conditional_testing.py

@@ -153,7 +153,7 @@ def __init__(self, file: str, tags: Set[str]):
             )
             tags = output.split()
 
-            want = test_case.tags
+            want = set(list(test_case.tags) + ["always"])
             assert want == set(tags), f"file {test_case.file}, want {want}, got {tags}"