test/rubygems/test_gem.rb: early failure when there is /tmp/Gemfile

Some test cases in rubygems assume that /tmp/Gemfile does not exist.
If it does, they fail with very difficult-to-understand message:

```
[ 149/2108] TestGemBundlerVersionFinder#test_bundler_version_with_bundle_update_bundler = 0.00
  1) Failure:
TestGemBundlerVersionFinder#test_bundler_version_with_bundle_update_bundler [/home/mame/work/ruby/test/rubygems/test_gem_bundler_version_finder.rb:38]:
Expected Gem::Version.new("2.0.2") to be nil.
```

I spent one hour to debug this issue.  To prevent the same accident,
this change makes the test suite stop when /tmp/Gemfile explicitly.

0 files changed, 0 insertions, 0 deletions