test_math.rb: rename check as assert_float

* test/ruby/test_math.rb (assert_float): rename with `assert_`
  prefix so that the caller frame would appear in backtraces.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45804 b2dd03c8-39d4-4d8f-98ff-823fe69b080e