Fix stale usage text and add missing harness to test/README.md

This brings the README's documentation of run_tests.sh up to date with
the actual entrypoint instead of the old run_generator_tests.sh text.

- The "More help" block pasted run_generator_tests.sh's usage output;
  replaced it with the real output of run_tests.sh --help.
- Listed test/tests/resolve_tag.sh alongside the other offline
  harnesses.
- Added a note to run the checkout's own test/run_tests.sh, not
  ${O2DPG_ROOT}/test/run_tests.sh, when the change under test is to a
  test script itself, since the latter resolves its sub-scripts next
  to itself and would test the released copy instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

test/README.md

@@ -58,6 +58,12 @@ If you are inside the source directory, you can simply run
 ```bash
 ${O2DPG_ROOT}/test/run_tests.sh [--fail-immediately] [--keep-artifacts] [SUBTEST...]
 ```
+If the change you are testing is to a test script itself (`run_tests.sh` or
+any `run_*_tests.sh`), invoke the checkout's own entrypoint instead, e.g.
+`bash test/run_tests.sh` from inside the checkout: `run_tests.sh` finds its
+sub-scripts next to itself, so calling `${O2DPG_ROOT}/test/run_tests.sh`
+tests the *released* copy of the script you just edited, not your change,
+even with `O2DPG_TEST_REPO_DIR` pointed at the checkout.
 
 ### Running a subset
 
@@ -68,12 +74,14 @@ some of them, name them:
 ${O2DPG_ROOT}/test/run_tests.sh generator relval
 ```
 
-The offline harnesses under `test/tests/` check the entrypoint's selection and
-exit-code aggregation without needing an O2 environment:
+The offline harnesses under `test/tests/` check the entrypoint's selection,
+exit-code aggregation and O2PDPSuite tag resolution without needing an O2
+environment:
 
 ```bash
 bash test/tests/run_tests_selection.sh
 bash test/tests/exit_code_aggregation.sh
+bash test/tests/resolve_tag.sh
 ```
 
 ### Keeping all test artifacts
@@ -91,26 +99,22 @@ ${O2DPG_ROOT}/test/run_tests.sh -h
 ```
 which will give you
 ```
-usage: run_tests.sh [--fail-immediately] [--keep-artifacts]
+
+usage: run_tests.sh [--fail-immediately] [--keep-artifacts] [SUBTEST...]
+
+  SUBTEST : one or more of: generator workflow relval (default: all)
 
   FLAGS:
 
-  --fail-immediately : abort as soon as the first tests fails
-  --keep-artifacts : keep simulation and tests artifacts, by default everything but the logs is removed after each test
+  --fail-immediately : stop after the first failing sub-test
+  --keep-artifacts   : keep simulation artifacts, not just the logs
 
   ENVIRONMENT VARIABLES:
 
-  O2DPG_TEST_REPO_DIR : Point to the source repository you want to test.
-  O2DPG_TEST_HASH_BASE : The base hash you want to use for comparison (optional)
-  O2DPG_TEST_HASH_HEAD : The head hash you want to use for comparison (optional)
-
-  If O2DPG_TEST_HASH_BASE is not set, it will be looked for ALIBUILD_BASE_HASH.
-  If also not set, this will be set to HEAD~1. However, if there are unstaged
-  changes, it will be set to HEAD.
+  O2DPG_TEST_REPO_DIR  : the source repository to test
+  O2DPG_TEST_HASH_BASE : base hash for the changed-file diff (optional)
+  O2DPG_TEST_HASH_HEAD : head hash for the changed-file diff (optional)
 
-  If O2DPG_TEST_HASH_HEAD is not set, it will be looked for ALIBUILD_HEAD_HASH.
-  If also not set, this will be set to HEAD. However, if there are unstaged
-  changes, it will left blank.
 ```
 
 ## When your change needs an unreleased O2