prevent jest leaking into the prod build

[mshustov]

@qn895 stumbled upon the problem that when you import lens types in a plugin, x-pack/test project fails type check. It's turned out that jest types leak into the global space and conflict with mocha types declared in FTR.
Also, I suspect jest might be built into the lens distributable.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: fc870a3

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
lens	479	478	-1

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
lens	877.5KB	876.8KB	-637.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[wylieconlon]

Did not run the code locally, but this appears safe. LGTM