DevTools: split slow formatter breakpoints tests

NOTRY=true
TBR=vsevik
BUG=357004

Review URL: https://codereview.chromium.org/213383010

git-svn-id: svn://svn.chromium.org/blink/trunk@170158 bbb929c8-8fbe-4397-9dbb-9b2b20218538