Add presubmit check to ensure that all fuzzers added are tested in CI (#819)
Add presubmit check to ensure that all fuzzers added are tested in CI (#819)

This patch should add a presubmit check to make sure presubmit works properly :-)
Specifically it ensures that any fuzzer added, is built/test-run in CI.
This patch was made in response to 
https://github.com/google/fuzzbench/pull/816#issuecomment-707397321
The only drawback I can think of slightly more annoyance for private experiments who will now be told
to edit fuzzers.yml even though it is useless for private experiments.
google/fuzzbench,google/fuzzbench,google/fuzzbench,google/fuzzbench,google/fuzzbench