test_bignum.rb: refine test
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_bignum.rb: refine test
merge with assert_interrupt, separate Bignum power from to_s, get
rid of busy loop, and skip if the conversion is too fast.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e