Add Bundler tests.

* Add `make test-bundler-parallel` executed on the upstream CI.
  See <41a48535d1/.github/workflows/ubuntu.yml (L22)>.
* The tests are disabled as a default, as it requires the internet.
  Run the Bundler tests as follows.
  ```
  $ mock --with bundler_tests ruby-3.1.0-161.fc37.src.rpm
  ```
* Fix a test for `bin/bundle update --bundler` in `make test-bundler`.

ruby-bundler-2.4.0-bundle-update-bundler-test-in-ruby.patch

@@ -0,0 +1,31 @@
+From bfa2f72cfa3bfde34049d26dcb24976316074ad7 Mon Sep 17 00:00:00 2001
+From: Jun Aruga <jaruga@redhat.com>
+Date: Mon, 21 Mar 2022 15:36:51 +0100
+Subject: [PATCH] Fix a test for `bin/bundle update --bundler` to pass on
+ ruby/ruby.
+
+Consider the case that the latest Bundler version on RubyGems is higher than
+the `system_bundler_version` (= `Bundler::VERSION`) in `make test-bundler` on
+ruby/ruby.
+
+See <https://bugs.ruby-lang.org/issues/18643>.
+---
+ spec/bundler/commands/binstubs_spec.rb | 5 ++++-
+ 1 file changed, 4 insertions(+), 1 deletion(-)
+
+diff --git a/spec/bundler/commands/binstubs_spec.rb b/spec/bundler/commands/binstubs_spec.rb
+index 198226207bc..2634f43417c 100644
+--- a/spec/bundler/commands/binstubs_spec.rb
++++ b/spec/bundler/commands/binstubs_spec.rb
+@@ -226,7 +226,10 @@
+ 
+         it "calls through to the latest bundler version" do
+           sys_exec "bin/bundle update --bundler", :env => { "DEBUG" => "1" }
+-          expect(out).to include %(Using bundler #{system_bundler_version}\n)
++          using_bundler_line = /Using bundler ([\w\.]+)\n/.match(out)
++          expect(using_bundler_line).to_not be_nil
++          latest_version = using_bundler_line[1]
++          expect(Gem::Version.new(latest_version)).to be >= Gem::Version.new(system_bundler_version)
+         end
+ 
+         it "calls through to the explicit bundler version" do

ruby.spec

@@ -85,6 +85,8 @@
 %bcond_without gmp
 %bcond_without hostname
 %bcond_without systemtap
+# Enable test when building on local.
+%bcond_with bundler_tests
 
 %if 0%{?fedora}
 %bcond_without hardening_test
@@ -161,6 +163,10 @@ Patch7: ruby-3.1.0-Don-t-query-RubyVM-FrozenCore-for-class-path.patch
 # Avoid possible timeout errors in TestBugReporter#test_bug_reporter_add.
 # https://bugs.ruby-lang.org/issues/16492
 Patch19: ruby-2.7.1-Timeout-the-test_bug_reporter_add-witout-raising-err.patch
+# Fix a test for `bin/bundle update --bundler` in `make test-bundler`.
+# https://bugs.ruby-lang.org/issues/18643
+# https://github.com/rubygems/rubygems/commit/bfa2f72cfa3bfde34049d26dcb24976316074ad7
+Patch20: ruby-bundler-2.4.0-bundle-update-bundler-test-in-ruby.patch
 
 Requires: %{name}-libs%{?_isa} = %{version}-%{release}
 Suggests: rubypick
@@ -186,6 +192,8 @@ BuildRequires: multilib-rpm-config
 BuildRequires: gcc
 BuildRequires: make
 BuildRequires: zlib-devel
+# The bundler/spec/runtime/setup_spec.rb requires the command `man`.
+%{?with_bundler_tests:BuildRequires: %{_bindir}/man}
 
 # This package provides %%{_bindir}/ruby-mri therefore it is marked by this
 # virtual provide. It can be installed as dependency of rubypick.
@@ -623,6 +631,7 @@ rm -rf ext/fiddle/libffi*
 %patch6 -p1
 %patch7 -p1
 %patch19 -p1
+%patch20 -p1
 
 # Provide an example of usage of the tapset:
 cp -a %{SOURCE3} .
@@ -938,6 +947,8 @@ mv test/fiddle/test_import.rb{,.disable}
 %{?test_timeout_scale:RUBY_TEST_TIMEOUT_SCALE="%{test_timeout_scale}"} \
   make check TESTS="-v $DISABLE_TESTS" MSPECOPT="-fs $MSPECOPTS"
 
+%{?with_bundler_tests:make test-bundler-parallel}
+
 %files
 %license BSDL
 %license COPYING