[rubygems/rubygems] Clear `RUBY_CODESIGN` env var while running tests

The `RUBY_CODESIGN` environment variable is used by mkmf-generated
Makefile to sign extension bundles on macOS. The variable specifies a
key identifier to use for signing given by the user. However, the key
is usually stored in `$HOME/Library/Keychains` directory, and the test
suite creates a fake `$HOME` directory. This causes the test suite to
try to find the specified key from the fake home directory, which
results in a failure.

https://github.com/rubygems/rubygems/commit/ddcfc65bf7

spec/bundler/support/rubygems_ext.rb

@@ -38,6 +38,10 @@ module Spec
       FileUtils.mkdir_p(Path.tmpdir)
 
       ENV["HOME"] = Path.home.to_s
+      # Remove "RUBY_CODESIGN", which is used by mkmf-generated Makefile to
+      # sign extension bundles on macOS, to avoid trying to find the specified key
+      # from the fake $HOME/Library/Keychains directory.
+      ENV.delete "RUBY_CODESIGN"
       ENV["TMPDIR"] = Path.tmpdir.to_s
 
       require "rubygems/user_interaction"

test/rubygems/helper.rb

@@ -351,6 +351,10 @@ class Gem::TestCase < Test::Unit::TestCase
     Dir.chdir @tempdir
 
     ENV["HOME"] = @userhome
+    # Remove "RUBY_CODESIGN", which is used by mkmf-generated Makefile to
+    # sign extension bundles on macOS, to avoid trying to find the specified key
+    # from the fake $HOME/Library/Keychains directory.
+    ENV.delete "RUBY_CODESIGN"
     Gem.instance_variable_set :@config_file, nil
     Gem.instance_variable_set :@user_home, nil
     Gem.instance_variable_set :@config_home, nil