rename test files with test_prefix

update rakefile

test/minitest_helper.rb

@@ -1,6 +1,5 @@
 require "minitest/autorun"
-
-require_relative "../lib/voicemeeter"
+require "voicemeeter"
 
 class Test
   class << self

test/test_error.rb

@@ -1,5 +1,5 @@
 require_relative "minitest_helper"
-require_relative "../lib/voicemeeter"
+require "voicemeeter"
 
 class CAPIErrorTest < Minitest::Test
   def test_it_raises_an_unknown_parameter_error