DevTools: Split slow script-formatter-breakpoints.html test into two tests.

BUG=247466
R=apavlov@chromium.org
TBR=apavlov

Review URL: https://codereview.chromium.org/188443004

git-svn-id: svn://svn.chromium.org/blink/trunk@168622 bbb929c8-8fbe-4397-9dbb-9b2b20218538