CI: assert_separately fails in Ruby 2.x tests

[olleolleolle]

Errors like this with assert_separately are currently happening. Here, even with a single argument in its list of arguments.

=> 123:     assert_separately(%w[-rcgi/escape], "#{<<-"begin;"}\n#{<<-"end;"}")
     124:     begin;
     125:       assert_equal("", CGI.unescapeURIComponent(''))
     126:     end;
===============================================================================
===============================================================================
Failure: test_cgi_unescape_accept_charset(CGIEscapeTest):
  assert_separately failed with error message
  pid 4022 exit 1
  | /Users/runner/hostedtoolcache/Ruby/2.6.10/arm64/bin/ruby: invalid option -:  (-h will show valid options) (RuntimeError)
  .
  <false> is not true.

[hsbt]

test-unit/test-unit#335 and test-unit/test-unit#337 are resolved this.