DEV: Add a new way to run specs in parallel with better output (#7778)

* DEV: Add a new way to run specs in parallel with better output

This commit:

 1. adds a new executable, `bin/interleaved_rspec` which works much like
    `rspec`, but runs the tests in parallel.

 2. adds a rake task, `rake interleaved:spec` which runs the whole test
    suite.

 3. makes autospec use this new wrapper by default. You can disable this
    by running `PARALLEL_SPEC=0 rake autospec`.

It works much like the `parallel_tests` gem (and relies on it), but
makes each subprocess use a machine-readable formatter and parses this
output in order to provide a better overall summary.

(It's called interleaved, because parallel was taken and naming is
hard).

* Make popen3 invocation safer

* Use FileUtils instead of shelling out

* DRY up reporter

* Moved summary logic into Reporter

* s/interleaved/turbo/g

* Move Reporter into its own file

* Moved run into its own class

* Moved Runner into its own file

* Move JsonRowsFormatter under TurboTests

* Join on threads at the end

* Acted on feedback from eviltrout

bin/turbo_rspec

@@ -0,0 +1,47 @@
+#!/usr/bin/env ruby
+
+require './lib/turbo_tests'
+require 'optparse'
+
+requires = []
+formatters = []
+
+OptionParser.new do |opts|
+  opts.on("-r", "--require PATH", "Require a file.") do |filename|
+    requires << filename
+  end
+
+  opts.on("-f", "--format FORMATTER", "Choose a formatter.") do |name|
+    formatters << {
+      name: name,
+      outputs: []
+    }
+  end
+
+  opts.on("-o", "--out FILE", "Write output to a file instead of $stdout") do |filename|
+    if formatters.empty?
+      formatters << {
+        name: "progress",
+        outputs: []
+      }
+    end
+    formatters.last[:outputs] << filename
+  end
+end.parse!(ARGV)
+
+requires.each { |f| require(f) }
+
+if formatters.empty?
+  formatters << {
+    name: "progress",
+    outputs: []
+  }
+end
+
+formatters.each do |formatter|
+  if formatter[:outputs].empty?
+    formatter[:outputs] << '-'
+  end
+end
+
+TurboTests::Runner.run(formatters, ARGV)