test_regexp.rb: assert prefix

* test/ruby/test_regexp.rb: rename and prefix `assert_` to show
  caller method names in backtrace.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46476 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_regexp.rb: split test_once

* test/ruby/test_regexp.rb (test_once): split for each cases than
  separating by comments.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46477 b2dd03c8-39d4-4d8f-98ff-823fe69b080e