Add lantern testing packages to npmignore

Bug: 451502260
Change-Id: I8ac348a273c8ae88b5c0a077d93c712e5eeb8d72
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7099141
Commit-Queue: Simon Zünd <[email protected]>
Reviewed-by: Simon Zünd <[email protected]>
Auto-Submit: Alex Rudenko <[email protected]>

Author: OrKoN

.npmignore

@@ -32,3 +32,4 @@ codereview.settings
 /front_end/legacy_test_runner
 /front_end/panels/timeline/fixtures/traces
 /front_end/core/i18n/locales/*.json
+/front_end/models/trace/lantern/testing