Add check to run-bindings-tests for excess files

Renaming or deleting IDL test cases can leave cruft, b/c we don't check
that all files are actually generated by run-bindings-tests.
For example, this required the following recent CL:
Remove V8TestSerializedScriptValueInterface.cpp
https://codereview.chromium.org/107093008

This adds a check to avoid this in future.
Also some associated refactoring of run-bindings-tests.

Review URL: https://codereview.chromium.org/112463002

git-svn-id: svn://svn.chromium.org/blink/trunk@163679 bbb929c8-8fbe-4397-9dbb-9b2b20218538