Make the isolated tests run on JRuby

As there is no forking on JRuby, we need to spawn sub-processes to make
the tests run in isolation.

Previously, we were defining globally env variables and running the test
file through backticks and delete these variables once the test ran.

Now, we simply rely on IO.popen as this is cross-platform and the env
variables are available during the child-process execution only so there
are no race conditions.

[Ben Browning & Robin Dupret]

activesupport/lib/active_support/testing/isolation.rb

@@ -70,14 +70,24 @@ def run_in_isolation(&blk)
             exit!
           else
             Tempfile.open("isolation") do |tmpfile|
-              ENV["ISOLATION_TEST"]   = self.class.name
-              ENV["ISOLATION_OUTPUT"] = tmpfile.path
+              env = {
+                ISOLATION_TEST: self.class.name,
+                ISOLATION_OUTPUT: tmpfile.path
+              }
 
               load_paths = $-I.map {|p| "-I\"#{File.expand_path(p)}\"" }.join(" ")
-              `#{Gem.ruby} #{load_paths} #{$0} #{ORIG_ARGV.join(" ")}`
+              orig_args = ORIG_ARGV.join(" ")
+              test_opts = "-n#{self.class.name}##{self.name}"
+              command = "#{Gem.ruby} #{load_paths} #{$0} #{orig_args} #{test_opts}"
 
-              ENV.delete("ISOLATION_TEST")
-              ENV.delete("ISOLATION_OUTPUT")
+              # IO.popen lets us pass env in a cross-platform way
+              child = IO.popen([env, command])
+
+              begin
+                Process.wait(child.pid)
+              rescue Errno::ECHILD # The child process may exit before we wait
+                nil
+              end
 
               return tmpfile.read.unpack("m")[0]
             end