chore: use git submodules for conformance tests (cloudevents#427)

I don't think downloading to `/tmp` for each `npm test` is such a great
idea. This does mean that contributors to this repo will need to run the
following command once on their clone after this commit lands.

```
git submodule init
git submodule update
```

Signed-off-by: Lance Ball <lball@redhat.com>

Author: lance

.github/workflows/nodejs-ci-action.yml

@@ -32,6 +32,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        submodules: true
     - name: Generate coverage report
       uses: actions/setup-node@v1
       with:

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "conformance"]
+	path = conformance
+	url = git@github.com:cloudevents/conformance.git