add integration test to verify cache is written if a builder fails

lib/rscons/cache.rb

@@ -2,7 +2,6 @@ require "digest/md5"
 require "fileutils"
 require "json"
 require "set"
-require "singleton"
 require "rscons/version"
 
 module Rscons
@@ -51,7 +50,6 @@ module Rscons
   #     },
   #   }
   class Cache
-    include Singleton
 
     # Name of the file to store cache information in
     CACHE_FILE = ".rsconscache"
@@ -59,6 +57,18 @@ module Rscons
     # Prefix for phony cache entries.
     PHONY_PREFIX = ":PHONY:"
 
+    class << self
+      # Access the singleton instance.
+      def instance
+        @instance ||= Cache.new
+      end
+
+      # Reset the cache (for unit/integration test purposes)
+      def reset!
+        @instance = nil
+      end
+    end
+
     # Create a Cache object and load in the previous contents from the cache
     # file.
     def initialize