(core) fix grist-core build after chai-as-promised change

Summary:
The docker build touches very little of the test directory, but
just enough to have broken after a chai-as-promised related refactor.
This adds a file that is sufficient to get the build rolling again.

Test Plan:
confirmed this change is sufficient to build functional
docker images

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3514

Dockerfile

@@ -29,6 +29,7 @@ RUN \
 COPY tsconfig.json /grist
 COPY tsconfig-ext.json /grist
 COPY test/tsconfig.json /grist/test/tsconfig.json
+COPY test/chai-as-promised.js /grist/test/chai-as-promised.js
 COPY app /grist/app
 COPY stubs /grist/stubs
 COPY buildtools /grist/buildtools