Deflake svg/custom/textPath-change-reference2.svg

This is a speculative fix for the flakiness we are seeing on the
textPath-change-reference2.svg test[1]. The failure is not reproducable
locally but appears as if runTest() is never called. This patch
refactors the test to not depend on runTest().

In addition, the test ids have been refactored to be understandable.

[1] http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&showAllRuns=true&showExpectations=true&tests=svg%2Fcustom%2FtextPath-change-reference2.svg

BUG=384566
TBR=dpranke

Review URL: https://codereview.chromium.org/333063003

git-svn-id: svn://svn.chromium.org/blink/trunk@176137 bbb929c8-8fbe-4397-9dbb-9b2b20218538