Backport rails/rails-perftest#2 to fix rake test:benchmark
clear ARGV to prevent mintest autorun errors:

Minitest expects the first argument in `ARGV` to be the path to a test file.
Because `rails benchmarker` and `rails profiler` define an on-the-fly test-case,
using the first `ARGV` to pass the code to execute this results in:

```
/Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:167:in `block in non_options': file not found: 1000.times{"a string"} (ArgumentError)
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:146:in `map!'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:146:in `non_options'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:207:in `non_options'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:52:in `process_args'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/minitest/unit.rb:891:in `_run'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/minitest/unit.rb:884:in `run'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:21:in `run'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:326:in `block (2 levels) in autorun'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:27:in `run_once'
	from /Users/senny/.rbenv/versions/1.9.3-p374/lib/ruby/1.9.1/test/unit.rb:325:in `block in autorun'
```

clearing ARGV after defining the test-case solves this issue.
Merge pull request #9173 from senny/backport_perftest_fix

Backport rails/rails-perftest#2 to fix rake test:benchmark