Include mustache spec tests in CI by checking out submodules

Our test suite already has support for running mustache specs,
but looks like we've forgotten about them in CI for many years now.

.github/workflows/verify.yml

@@ -11,7 +11,8 @@ jobs:
         node-version: [8.x, 10.x, 12.x, 14.x, 15.x]
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
+        submodules: recursive
       - name: Setup Node.js
         uses: actions/setup-node@v1
         with:
@@ -29,7 +30,8 @@ jobs:
         node-version: [0.10.x, 0.12.x, 4.x, 6.x]
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
+        submodules: recursive
       - name: Setup Node.js
         uses: actions/setup-node@v1
         with: