Add minitest-proveit to force assertions

[teoljungberg]

This PR adds minitest-proveit to force that all tests executes some kind
of assertion, and if they do not - they will fail automatically.

[teoljungberg]

Example:

..b.com/fxn/zeitwerk fix-spelling-errors % git diff
diff --git a/test/lib/test_inflector.rb b/test/lib/test_inflector.rb
index 7a0af7d..13cdc1a 100644
--- a/test/lib/test_inflector.rb
+++ b/test/lib/test_inflector.rb
@@ -39,4 +39,8 @@ class TestInflector < Minitest::Test

     assert_equal "UsersController", inflector.camelize("users_controller", nil)
   end
+
+  test "no assertions fail" do
+    2 + 4 == 0
+  end
 end
..b.com/fxn/zeitwerk fix-spelling-errors % bundle exec rake

# Running tests with run options --seed 17253:

.........................................................................................................................................................................................................................................................................................................................................................................F.................

Finished tests in 2.280895s, 166.1628 tests/s, 394.5820 assertions/s.


Failure:
TestInflector#test_no_assertions_fail [/Users/teo/src/github.com/fxn/zeitwerk/.bundle/ruby/3.1.0/gems/minitest-5.17.0/lib/minitest/test.rb:107]
Minitest::Assertion: Absence of failure is not success. Prove it!

379 tests, 900 assertions, 1 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (1)

[fxn]

Seems like a good idea, let's give it a try at least.

Thanks!

[teoljungberg]

Since there were no test-cases without assertions it is another added development-dependency currently. But since zeitwerk can get contributions I felt that it was a good guard-railing to have.

It's at least worth a shot!