So I realized MiniTest had some incompatible updates from 4.x to 5.x And somehow I was using version 5.x but my problem seems to occur when 4.x is used.
Here is when it works as expected:
irb> require 'minitest'
=> true
irb> require 'minitest/autorun'
=> true
irb> MiniTest::Test
=> Minitest::Test
irb> MiniTest.constants
=> [:Parallel, :VERSION, :Runnable, :AbstractReporter, :Reporter, :ProgressReporter, :StatisticsReporter, :SummaryReporter, :CompositeReporter, :Assertion, :Skip, :UnexpectedError, :Guard, :BacktraceFilter, :Test, :Assertions, :Unit, :Spec, :Expectations, :Mock]
irb> MiniTest::Unit::VERSION
=> "5.3.2"
Here is when it is not. I installed a fresh copy of Ruby 2.1 and reproduced the following.
irb> require 'minitest'
LoadError: cannot load such file -- minitest
irb> require 'minitest/autorun'
=> true
irb> MiniTest::Test
NameError: uninitialized constant MiniTest::Test
irb> MiniTest.constants
=> [:Assertion, :Skip, :BacktraceFilter, :Assertions, :Unit, :Spec, :Expectations, :Mock]
irb> MiniTest::Unit::VERSION
=> "4.7.5"
Summary
So I guess when an older minitest version is used, I should install version 5 explictly via Gemfile/bundler.