Add in-repo Complement test to sanity check Synapse version matches git checkout (#19476)

This way we actually detect problems like
https://github.com/element-hq/synapse/pull/19475 as they happen instead
of being invisible until something breaks.

Sanity check that Complement is testing against your code changes
(whether it be local or from the PR in CI).

```
COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh --in-repo -run TestSynapseVersion
```

.github/workflows/tests.yml

@@ -705,6 +705,13 @@ jobs:
           toolchain: ${{ env.RUST_VERSION }}
       - uses: Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5 # v2.8.2
 
+      # We use `poetry` in `complement.sh`
+      - uses: matrix-org/setup-python-poetry@5bbf6603c5c930615ec8a29f1b5d7d258d905aa4 # v2.0.0
+        with:
+          poetry-version: "2.1.1"
+          # Matches the `path` where we checkout Synapse above
+          working-directory: "synapse"
+
       - name: Prepare Complement's Prerequisites
         run: synapse/.ci/scripts/setup_complement_prerequisites.sh
 
@@ -713,6 +720,32 @@ jobs:
           cache-dependency-path: complement/go.sum
           go-version-file: complement/go.mod
 
+      # Run the image sanity check test first as this is the first thing we want to know
+      # about (are we actually testing what we expect?) and we don't want to debug
+      # downstream failures (wild goose chase).
+      - name: Sanity check Complement image
+        id: run_sanity_check_complement_image_test
+        # -p=1: We're using `-p 1` to force the test packages to run serially as GHA boxes
+        #     are underpowered and don't like running tons of Synapse instances at once.
+        # -json: Output JSON format so that gotestfmt can parse it.
+        #
+        # tee /tmp/gotest-complement.log: We tee the output to a file so that we can re-process it
+        # later on for better formatting with gotestfmt. But we still want the command
+        # to output to the terminal as it runs so we can see what's happening in
+        # real-time.
+        run: |
+          set -o pipefail
+          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh --in-repo -p 1 -json -run 'TestSynapseVersion/Synapse_version_matches_current_git_checkout' 2>&1 | tee /tmp/gotest-sanity-check-complement.log
+        shell: bash
+        env:
+          POSTGRES: ${{ (matrix.database == 'Postgres') && 1 || '' }}
+          WORKERS: ${{ (matrix.arrangement == 'workers') && 1 || '' }}
+
+      - name: Formatted sanity check Complement test logs
+        # Always run this step if we attempted to run the Complement tests.
+        if: always() && steps.run_sanity_check_complement_image_test.outcome != 'skipped'
+        run: cat /tmp/gotest-sanity-check-complement.log | gotestfmt -hide "successful-downloads,empty-packages"
+
       - name: Run Complement Tests
         id: run_complement_tests
         # -p=1: We're using `-p 1` to force the test packages to run serially as GHA boxes