brew test fails on 10.13 due to different assertion class

[mistydemeo]

Please always follow these steps:

• Confirmed this is a problem with running a brew command and not brew installing or the post-install behaviour of one or more formulae? If it's a formulae-specific problem please file this issue at https://github.com/Homebrew/homebrew-core/issues/new
• Ran brew update and retried your prior step?
• Ran brew doctor, fixed all issues and retried your prior step?

10.13 uses Ruby 2.3 as its system Ruby, with some differences in how Test::Unit's assertions work. This causes some troubles with brew test, which uses the system Ruby's Test::Unit.

Specifically, the assertions in the system Ruby 2.0 on older OSs raise a MiniTest::Assertion on assertion failure, while Ruby 2.3 in 10.13 raises a Test::Unit::AssertionFailedError. Since the MiniTest constant isn't loaded at all, brew test fails with an uninitialized constant; even if it were these exceptions can't be rescued via MiniTest::Assertion.

Steps to reproduce:

1. Find a formula with a failing assertion (creating one if necessary).
2. Run brew test formulaname

Expected:

The failed test is reported as expected.

Actual:

A Error: uninitialized constant Homebrew::MiniTest error is printed.

cc @MikeMcQuaid