Replace MiniTest with Minitest

- MiniTest constant is not declared anymore

lib/minitest/reporters/base_reporter.rb

@@ -126,7 +126,7 @@ def print_info(e, name = true)
 
         # When e is a Minitest::UnexpectedError, the filtered backtrace is already part of the message printed out
         # by the previous line. In that case, and that case only, skip the backtrace output.
-        return if e.is_a?(MiniTest::UnexpectedError)
+        return if e.is_a?(Minitest::UnexpectedError)
 
         trace = filter_backtrace(e.backtrace)
         trace.each { |line| print_with_info_padding(line) }