tests: support "rake rcov" if rcov is installed

If the rcov gem is installed, `rake rcov` will generate a test coverage
report in the coverage directory.

I picked rcov because it is 1.8 compatible. But it could easily be
swapped out for another coverage tool.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/test/.gitignore

@@ -0,0 +1 @@
+/coverage

Library/Homebrew/test/Rakefile

@@ -1,9 +1,10 @@
 require 'rake'
 require 'rake/testtask'
 
-Dir.chdir File.expand_path(File.dirname(__FILE__))
+TEST_DIRECTORY = File.expand_path(File.dirname(__FILE__))
+TEST_FILES = FileList["#{TEST_DIRECTORY}/test_*.rb"]
 
-TEST_FILES = FileList['test_*.rb']
+Dir.chdir TEST_DIRECTORY
 
 task :default => :test
 
@@ -21,3 +22,20 @@ namespace :test do
     end
   end
 end
+
+begin
+  require 'rubygems'
+  require 'rcov/rcovtask'
+
+  Rcov::RcovTask.new do |t|
+    t.libs << Dir.pwd
+    t.test_files = TEST_FILES
+    t.rcov_opts = %w{--exclude=Gems
+                     --exclude=test_
+                     --exclude=testball
+                     --exclude=testing}
+    t.output_dir = TEST_DIRECTORY+'/coverage'
+  end
+rescue LoadError
+  nil
+end

Library/Homebrew/test/testing_env.rb

@@ -25,8 +25,13 @@ HOMEBREW_CURL_ARGS = '-fsLA'
 MACOS_VERSION=10.6
 
 (HOMEBREW_PREFIX+'Library/Formula').mkpath
+
+prevwd = Dir.pwd
 Dir.chdir HOMEBREW_PREFIX
-at_exit { HOMEBREW_PREFIX.parent.rmtree }
+at_exit do
+  Dir.chdir prevwd
+  HOMEBREW_PREFIX.parent.rmtree
+end
 
 # Test fixtures and files can be found relative to this path
 TEST_FOLDER = Pathname.new(ABS__FILE__).parent.realpath