Add README.md for TAP test groups directory

Documents groups.json format (one entry per line, compact arrays),
group naming conventions, directory structure, and infrastructure-free
groups.

test/tap/groups/README.md

@@ -0,0 +1,56 @@
+# TAP Test Groups
+
+This directory organizes TAP tests into **groups** that share the same infrastructure and configuration.
+
+## groups.json
+
+`groups.json` maps each test binary to one or more groups. Every entry is a test name (without path) and an array of group names it belongs to.
+
+### Format
+
+One entry per line, compact arrays with spaces around `:` and inside `[ ]`:
+
+```json
+{
+  "admin-listen_on_unix-t" : [ "legacy-g1","mysql84-g1","mysql90-g1" ],
+  "auth_unit-t" : [ "unit-tests-g1" ],
+  "basic-t" : [ "legacy-g1","mysql84-g1" ]
+}
+```
+
+This format keeps `grep` effective for quick lookups:
+
+```bash
+# Which groups does a test belong to?
+grep "my_test-t" groups.json
+
+# Which tests are in a group?
+grep "unit-tests-g1" groups.json
+```
+
+Do **not** reformat the file with multi-line arrays or change the indentation style.
+
+### Group naming convention
+
+Groups follow the pattern `<base>-g<N>` where:
+
+- `<base>` is the group name (e.g., `legacy`, `mysql84`, `unit-tests`)
+- `<N>` is a subgroup number for splitting tests into parallel batches
+
+The system strips the `-gN` suffix to find the base group directory for `infras.lst`, `env.sh`, and hooks.
+
+## Group directory structure
+
+Each group has a directory under `test/tap/groups/<base>/` with some or all of:
+
+| File | Purpose |
+|------|---------|
+| `env.sh` | Environment variables for the group (sourced early by the runner) |
+| `infras.lst` | Required backend infrastructures (one per line) |
+| `setup-infras.bash` | Hook: runs after all backends are up |
+| `pre-proxysql.bash` | Hook: runs before test execution (e.g., cluster setup) |
+| `pre-cleanup.bash` | Hook: runs after tests, before container teardown |
+
+## Infrastructure-free groups
+
+Groups that set `SKIP_PROXYSQL=1` in their `env.sh` run without ProxySQL or any backend containers. The `unit-tests` group is the primary example. See `test/infra/README.md` section 6.1 for details.