Enable warnings when running tests

This PR enables rake's test task to opt into running the test suite with
warnings, this is to avoid warnings sneaking into the library.

Illustrative example:

```
% git diff
diff --git a/lib/zeitwerk.rb b/lib/zeitwerk.rb
index a1335d1..615d516 100644
--- a/lib/zeitwerk.rb
+++ b/lib/zeitwerk.rb
@@ -16,6 +16,7 @@ module Zeitwerk
   # @experimental
   # @sig () -> void
   def self.with_loader
+    a = 1
     loader = Zeitwerk::Loader.new
     yield loader
   ensure
% bundle exec rake
/Users/teo/src/github.com/fxn/zeitwerk/lib/zeitwerk.rb:19: warning: assigned but unused variable - a
... output omitted ...
```

Rakefile

@@ -7,4 +7,5 @@ task :default => :test
 Rake::TestTask.new do |t|
   t.test_files = Dir.glob('test/lib/**/test_*.rb')
   t.libs << "test"
+  t.warning = true
 end