[ruby/rubygems] If we use shared GEM_HOME and install multiple versions, it may cause unexpected test failures.

```
       Fetching gem metadata from https://gem.repo4/.
       Resolving dependencies...
       Resolving dependencies...
       # $? => 0
       cannot load such file -- diff/lcs
```

ruby/rubygems@668b300261

Author: hsbt

spec/bundler/spec_helper.rb

@@ -19,6 +19,10 @@
 ENV["BUNDLER_EDITOR"] = nil
 require "bundler"
 
+# If we use shared GEM_HOME and install multiple versions, it may cause
+# unexpected test failures.
+gem "diff-lcs"
+
 require "rspec/core"
 require "rspec/expectations"
 require "rspec/mocks"