build: Lower default Karma logLevel during local development

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