build: Lower default Karma logLevel during local development
Follows-up
fa63a70ae90 which enabled this for CI to investigate
a Jenkins-specific issue, but it makes for a very noisy and visually
unpleasant local experience because these Karma lines interweave
with the structural output from the qunit/karma reporter.
With this change applied, the local experience from 'npm run qunit'
has clean output again like so:
```
…
mediawiki.storage
✔ set/get(Object) with storage support
✔ set/get(Object) with storage methods disabled
✔ set/get(Object) with storage object disabled
mediawiki.template
✔ add
✔ compile
✔ get
Finished in 13 secs
SUMMARY:
✔ 440 tests completed
```
Change-Id: I21555468ed30ca3a2220eba1d8a78cc140a13399