It was returning old file coverages as empty arrays to the user.
  [ruby-core:74596] [Bug #12220]
* ext/coverage/coverage.c (rb_coverages): remove unused static state.
* thread.c: Moved and renamed coverage_clear_result_i to reset_coverage_i.
* test/coverage/test_coverage.rb: improve precision of tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54504 b2dd03c8-39d4-4d8f-98ff-823fe69b080e