Prevent void context warnings

The constant reference A::B used to trigger autoloading causes a warning
to be logged about the possible use of :: in a void context so assign it
to the _ variable to prevent the warnings from being triggered.

activesupport/test/autoloading_fixtures/raises_arbitrary_exception.rb

@@ -1,4 +1,4 @@
 RaisesArbitraryException = 1
-A::B # Autoloading recursion, also expected to be watched and discarded.
+_ = A::B # Autoloading recursion, also expected to be watched and discarded.
 
 raise Exception, 'arbitray exception message'

activesupport/test/autoloading_fixtures/throws.rb

@@ -1,4 +1,4 @@
 Throws = 1
-A::B # Autoloading recursion, expected to be discarded.
+_ = A::B # Autoloading recursion, expected to be discarded.
 
 throw :t