Revert "Fix tests for travis failures."

This reverts commit 61b5976959ad1b86a703fdc12b2a0a7787bad64e.
Re-do 61b5976959ad without indenting changes.
Fix invariant/sanity check code.

The original code broke on an odd number of running threads.
Don't use error-prone hardcoded syms.
Get rid of the ruby-atomic dependency.