require test_helper in all tests
Fix typo in filename
test helpers: assert_valid, refute_valid

Including partial conversion of the tests to using it in those cases
where it was a trivial emacs macro. There are many additional calls to
validation that I did not attempt to rewrite yet.
Use assert/refute valid in TestExtendedSchema
Switch to using MiniTest::Unit

This required only a few changes:

1. `assert_nothing_raised` has been removed; *every* test is asserting
   nothing was raised simply by running to completion.
2. `assert_raise` is `assert_raises`
3. `assert_not_equal` is `refute_equal`
Fix all warnings except `URI.escape` deprecation
Update new test to use test_helper
Extract common type validation tests
Extract minimum/maximum number validation tests
Extract common string property validation tests
Rename a shared tests module for consistency
Extract min/max items; include directly into DraftN test cases
Extract uniqueItems test
Extract items, additionalItems tests
Extract additionalProperties, patternProperties
Tidy up `format` tests
Use module namespaces for shared tests

The names were getting pretty unwieldy, and were about to get even
crazier for the extracted string format tests et al.
Extract multipleOf/divisibleBy tests
Extract most string format tests

I originally also put `test_format_datetime` into the base shared format
tests, but our draft4 validation is significantly different from
draft1..3. I've not yet determined whether that is correct; draft4
passes through ruby's `DateTime.rfc3339` method, while the others are
validated against a regexp claiming to match ISO-8601 format.
Use minitest gem, not stdlib version