Make run-bindings-tests clean up cache files

run-bindings-tests --reset-results
currently leaves cache files in the reference directory,
which then are picked up as excess files.
This fixes this by deleting these files.

In a follow-up (once the "run all compiles in one process" CL is done)
I'll just eliminate the cache generation (as it won't be necessary).

TBR=haraken

Review URL: https://codereview.chromium.org/196523004

git-svn-id: svn://svn.chromium.org/blink/trunk@168991 bbb929c8-8fbe-4397-9dbb-9b2b20218538