Improve rake tasks for specs

- clean coverage directory before running specs to avoid simplecov
  slowdowns
- add valgrind task to run valgrind; normal spec does not run it

Rakefile.rb

@@ -26,6 +26,16 @@ task :spec do |task, args|
     end
   end
 end
+task :spec => :clean
+
+task :valgrind do
+  begin
+    ENV["spec-valgrind"] = "1"
+    Rake::Task[:spec].execute
+  ensure
+    ENV.delete("spec-valgrind")
+  end
+end
 
 # dspec task is useful to test the distributable release script, but is not
 # useful for coverage information.
@@ -43,4 +53,4 @@ task :user_guide do
   system("ruby", "-Ilib", "rb/gen_user_guide.rb")
 end
 
-task :all => [:spec, :dspec, :user_guide]
+task :all => [:valgrind, :dspec, :user_guide]

spec/propane_spec.rb

@@ -119,7 +119,7 @@ EOF
     results = Results.new(stdout, stderr, status)
     # Valgrind is only reliably available on Linux, so limit the leak check to
     # Linux platforms.
-    if RUBY_PLATFORM =~ /linux/
+    if RUBY_PLATFORM =~ /linux/ && ENV["spec-valgrind"]
       stdout, stderr, status = Open3.capture3("valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose spec/run/testparser")
       vgout = stdout + stderr
       File.binwrite("spec/run/.vgout", vgout)