Trunk tracking tests: Fixes to pass the system tests

A few fixes to help making the trunk tracking update from r3715 to
r3736 pass the system tests with the native fetcher and under
valgrind.

- Native fetcher: fortify handling of content lenght (and absense)
- Native fetcher: fail when the stream terminates before having
                  read the headers
- Tests: Rename test_filter -> start_test in ngx_system_test.sh for
         a test
- Tests: Add blockingrewrite key for two tests that need it
- Tests: Update localhost -> 127.0.0.1. The native fetcher uses
         dns to resolve, and won't be able to retreive an ip for
         localhost
- Tests: Allow outstanding proxy fetches some time to finish
         when running under valgrind, before terminating nginx
- Valgrind: Add suppressions to make testing a resealse build pass

Should fix:
https://github.com/pagespeed/ngx_pagespeed/issues/609
Probably improves upon
https://github.com/pagespeed/ngx_pagespeed/pull/602