Make encoding-labels test easier to debug

It's hard to find out which label failed for a given encoding.
Revise encoding-labels.html to print out {encoding name, label) pair
instead of just 'encoding name' for each test run.

BUG=412053
TEST=fast/encoding/encoding-labels.html

Review URL: https://codereview.chromium.org/599403002

git-svn-id: svn://svn.chromium.org/blink/trunk@182685 bbb929c8-8fbe-4397-9dbb-9b2b20218538