Make a final "success" job for each CI test suite

- That job can be used in branch protection rules

.github/workflows/macos.yml

@@ -92,3 +92,10 @@ jobs:
           n=$(nproc)
           echo "Using $n test jobs"
           ${build_dir}/rippled --unittest --unittest-jobs $n
+
+  success:
+    name: mac test success
+    needs: test
+    runs-on: ubuntu-latest
+    steps:
+      - run: true

.github/workflows/nix.yml

@@ -163,6 +163,13 @@ jobs:
         run: |
           ${build_dir}/rippled --unittest --unittest-jobs $(nproc)
 
+  success:
+    name: nix test success
+    needs: test
+    runs-on: ubuntu-latest
+    steps:
+      - run: true
+
   coverage:
     strategy:
       fail-fast: false

.github/workflows/windows.yml

@@ -95,3 +95,10 @@ jobs:
         run: |
           ${build_dir}/${{ matrix.configuration.type }}/rippled --unittest \
               --unittest-jobs $(nproc)
+
+  success:
+    name: windows test success
+    needs: test
+    runs-on: ubuntu-latest
+    steps:
+      - run: true