Do not automatically require minitest-bisect

This is currently causing an issue in 6-1-stable CI due to a chain of
dependencies: minitest-bisect -> minitest-server -> drb. The last
working version of Drb for Ruby 2.5 and 2.6 includes usage of
ruby2_keywords but does not declare a dependency on the shim gem
(meaning that version of Drb actual does not work on those Ruby
versions).

ruby2_keywords was [added][1] to the 6-1-stable Active Support gemspec
to address the issue with Drb, but this only fixes cases where Drb is
required by Rails. However, there are still failing tests on 6-1-stable
due to Drb being loaded in integration tests where a dummy Rails
application calls Bundler.require and the above dependency chain is
required.

To address those test failures, this commit prevents minitest-bisect
from being automatically required when the whole bundle is required. It
is not strictly necessary for main or any stable branches other than
6-1-stable, but it seems more correct since minitest_bisect is ran as an
external command anyways.

[1]: 5a54e2f

Author: skipkayhil

Gemfile

@@ -132,7 +132,7 @@ local_gemfile = File.expand_path(".Gemfile", __dir__)
 instance_eval File.read local_gemfile if File.exist? local_gemfile
 
 group :test do
-  gem "minitest-bisect"
+  gem "minitest-bisect", require: false
   gem "minitest-ci", require: false
   gem "minitest-retry"