Added some tests around file uris

I was intrigued by how this works, and had no idea if it was already
being tested (it is, implicitly) so I figured I'd put some tests around
it.
Merge pull request #169 from iainbeeston/file_uris

Added some tests around file uris
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