Improve RUBYOPT's handling in tests

When debugging the issue related to ruby/irb#919,
I noticed that debugger tests don't respect the IRB version I specified
in the Gemfile. This is because console tests force override the RUBYOPT
env, which will remove the `-rbundler/setup` injected by bundler.

Further more, if tests use `run_rdbg` with the `rubyopt` option, the
RUBYOPT will be overridden yet again.

So in this commit I did 2 improvements:

1. `run_rdbg` should append instead of override RUBYOPT
2. If tests are executed with bundler, we also run the debugger in PTY
   process with bundler by appending `-rbundler/setup` to RUBYOPT

test/support/console_test_case.rb

@@ -120,7 +120,9 @@ def debug_code(program, remote: true, verbose: false, &test_steps)
     end
 
     def run_test_scenario cmd, test_info, verbose: false
-      PTY.spawn({ "HOME" => pty_home_dir }, cmd) do |read, write, pid|
+      env = { "HOME" => pty_home_dir }
+
+      PTY.spawn(env, cmd) do |read, write, pid|
         test_info.backlog = []
         test_info.last_backlog = []
         begin
@@ -225,7 +227,12 @@ def prepare_test_environment(program, test_steps, &block)
       @scenario = []
       test_steps.call
       @scenario.freeze
-      inject_lib_to_load_path
+
+      ENV['RUBYOPT'] = "-I #{__dir__}/../../lib"
+
+      if ENV.key?('BUNDLE_GEMFILE')
+        ENV["RUBYOPT"] += " -rbundler/setup"
+      end
 
       block.call
 
@@ -283,7 +290,7 @@ def run_rdbg program, options: nil, rubyopt: nil, &test_steps
       prepare_test_environment(program, test_steps) do
         test_info = TestInfo.new(dup_scenario, 'LOCAL', /\(rdbg\)/)
         cmd = "#{RDBG_EXECUTABLE} #{options} -- #{temp_file_path}"
-        cmd = "RUBYOPT=#{rubyopt} #{cmd}" if rubyopt
+        ENV["RUBYOPT"] += " #{rubyopt}" if rubyopt
         run_test_scenario cmd, test_info
       end
     end
@@ -301,10 +308,6 @@ def new_thread &block
       end
     end
 
-    def inject_lib_to_load_path
-      ENV['RUBYOPT'] = "-I #{__dir__}/../../lib"
-    end
-
     def assert_empty_queue test_info, exception: nil
       message = "Expected all commands/assertions to be executed. Still have #{test_info.queue.length} left."
       if exception