Make unit testing mode controlled by env var

This feels more user-defined and unlikely to be affected by future Bolt
product changes, compared to relying on whether or not a tty is present.

Author: reidmv

Diff for: spec/spec_helper_local.rb

@@ -2,6 +2,10 @@
 
 require 'puppet'
 
+# This environment variable can be read by Ruby Bolt tasks to prevent unwanted
+# auto-execution, enabling easy unit testing.
+ENV["RSPEC_UNIT_TEST_MODE"] ||= "TRUE"
+
 if Gem::Version.new(Puppet.version) >= Gem::Version.new('6.0.0')
   begin
     require 'bolt_spec/plans'

Diff for: tasks/puppet_infra_upgrade.rb

@@ -91,9 +91,10 @@ def wait_until_connected(nodes:, token_file:, timeout: 120)
   end
 end
 
-# Run the task if we got piped input. In order to enable unit testing, do not
-# run the task if input is a tty.
-unless STDIN.tty?
+# Run the task unless an environment flag has been set, signaling not to. The
+# environment flag is used to disable auto-execution and enable Ruby unit
+# testing of this task.
+unless ENV['RSPEC_UNIT_TEST_MODE']
   upgrade = PEAdm::Task::PuppetInfraUpgrade.new(JSON.parse(STDIN.read))
   upgrade.execute!
 end