extend timeout to 15 seconds for ARMv8 environment

	Show how many seconds it timeouts

	By this we can easily extend timeout.

	Extend timeout of test_clear_unreachable_keyword_args

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@61630 b2dd03c8-39d4-4d8f-98ff-823fe69b080e