clock_gettime_spec.rb: add more platform guards for RubyCI

`Errno::EINVAL: Invalid argument - clock_gettime` could occur even on Linux.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66633 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/process/clock_gettime_spec.rb

@@ -1,7 +1,10 @@
 require_relative '../../spec_helper'
 
 describe "Process.clock_gettime" do
-  platform_is_not :windows, :solaris do
+  # This does not succeed with some platforms:
+  # RHEL: https://rubyci.org/logs/rubyci.s3.amazonaws.com/rhel_zlinux/ruby-trunk/log/20181229T063303Z.fail.html.gz
+  # Solaris: https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11x/ruby-trunk/log/20181229T002409Z.fail.html.gz
+  platform_is_not :windows, :linux, :solaris do
     it 'can be called with all declared clocks' do
       Process.constants.select { |c| c.to_s.start_with?('CLOCK_') }.each do |c|
         Process.clock_gettime(Process.const_get(c)).should be_an_instance_of(Float)