Don't store test Proc if we might need to marshall the formula

Procs cannot be marshalled, but formula objects may be as part of the
BuildError exception. If the formula object has a reference to a Proc,
this will fail.

Work around it by not storing the test Procs during installations.

Library/Homebrew/formula.rb

@@ -800,7 +800,11 @@ private
 
     def test &block
       return @test unless block_given?
-      @test = block
+      # If this formula is the executing script, we are performing an
+      # install. Since it may be marshalled back to the main process as
+      # part of an exception, don't store this Proc because Procs can't
+      # be marshalled.
+      @test = block unless $0 != HOMEBREW_BREW_FILE.to_s
     end
   end
 end