Ignore calls in other threads when counting ruby calls for a test

[dylanahsmith]

Closes #113

Problem

As stated in #113 we have flaky test failures in a test that is counting ruby calls using a tracepoint.

I had trouble reproducing the test failure locally, so I commented out the test using TracePoint and started a tracepoint in test_helper.rb, which was able to capture calls on the Thread module we were seeing. After a bit of debugging, I found the source of the problem, that can be demonstrated using this in test/test_helper.rb

main_thread = Thread.current

c_call_trace = TracePoint.trace(:c_call) do |t|
  if t.defined_class == Thread
    thread_name = Thread.current == main_thread ? "main" : "other"
    raise "unexpected call to #{t.defined_class}##{t.method_id} on #{thread_name} thread"
  end
end

which outputs

Traceback (most recent call last):
	8: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest.rb:68:in `block in autorun'
	7: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest.rb:138:in `run'
	6: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:27:in `start'
	5: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:27:in `map'
	4: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:27:in `each'
	3: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:27:in `times'
	2: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:28:in `block in start'
	1: from /Users/dylants/.gem/ruby/2.5.8/gems/minitest-5.14.2/lib/minitest/parallel.rb:28:in `new'
/Users/dylants/src/liquid-c/test/test_helper.rb:10:in `block in <top (required)>': unexpected call to Thread#initialize on main thread (RuntimeError)

So these calls are coming from this line in minitest: https://github.com/seattlerb/minitest/blob/v5.14.2/lib/minitest/parallel.rb#L29

Solution

Filter calls to another thread in the test that was asserting on the number of ruby calls made.