[test] Better test failure reporting

[soutaro]

If you have a test which fails, the stdlib test reports like the following:

Minitest::Assertion: Expected ["[Dir.mktmpdir] ReturnTypeError: expected `::Integer` but returns `\"/var/folders/b9/sw1f2b996cd_tclwhhmh5tfc0000gn/T/d20200108-67558-4ri1a4\"`", "[Dir.mktmpdir] ReturnTypeError: expected `::Integer` but returns `\"/var/folders/b9/sw1f2b996cd_tclwhhmh5tfc0000gn/T/foo20200108-67558-2jmzq0bar\"`", "[Dir.mktmpdir] ReturnTypeError: expected `::Integer` but returns `\"/var/folders/b9/sw1f2b996cd_tclwhhmh5tfc0000gn/T/foo20200108-67558-315pdj\"`", "[Dir.mktmpdir] ReturnTypeError: expected `::Integer` but returns `\"/var/folders/b9/sw1f2b996cd_tclwhhmh5tfc0000gn/T/d20200108-67558-42bd5d\"`"] to be empty.

It tells us there is an inconsistent method call but does not tell which one is the source of the failure.

We need an improvement to help to identify the source of the failure.