Fix Resolv specs to not depend on a system /etc/hosts file

* https://rubyci.org/logs/rubyci.s3.amazonaws.com/archlinux/ruby-master/log/20190609T153804Z.fail.html.gz
* Thanks @naruse for the tip.

spec/ruby/library/resolv/fixtures/hosts

@@ -0,0 +1 @@
+127.0.0.1   localhost localhost4

spec/ruby/library/resolv/get_address_spec.rb

@@ -4,11 +4,11 @@ require 'resolv'
 describe "Resolv#getaddress" do
   platform_is_not :windows do
     it "resolves localhost" do
-      res = Resolv.new([Resolv::Hosts.new])
+      hosts = Resolv::Hosts.new(fixture(__FILE__ , "hosts"))
+      res = Resolv.new([hosts])
 
-      lambda {
-        res.getaddress("localhost")
-      }.should_not raise_error(Resolv::ResolvError)
+      res.getaddress("localhost").should == "127.0.0.1"
+      res.getaddress("localhost4").should == "127.0.0.1"
     end
   end
 

spec/ruby/library/resolv/get_addresses_spec.rb

@@ -4,11 +4,11 @@ require 'resolv'
 describe "Resolv#getaddresses" do
   platform_is_not :windows do
     it "resolves localhost" do
-      res = Resolv.new([Resolv::Hosts.new])
+      hosts = Resolv::Hosts.new(fixture(__FILE__ , "hosts"))
+      res = Resolv.new([hosts])
 
-      addresses = res.getaddresses("localhost")
-      addresses.should_not == nil
-      addresses.size.should > 0
+      res.getaddresses("localhost").should == ["127.0.0.1"]
+      res.getaddresses("localhost4").should == ["127.0.0.1"]
     end
   end
 end

spec/ruby/library/resolv/get_name_spec.rb

@@ -4,9 +4,10 @@ require 'resolv'
 describe "Resolv#getname" do
   platform_is_not :windows do
     it "resolves 127.0.0.1" do
-      lambda {
-        Resolv.getname("127.0.0.1")
-      }.should_not raise_error(Resolv::ResolvError)
+      hosts = Resolv::Hosts.new(fixture(__FILE__ , "hosts"))
+      res = Resolv.new([hosts])
+
+      res.getname("127.0.0.1").should == "localhost"
     end
   end
 

spec/ruby/library/resolv/get_names_spec.rb

@@ -4,11 +4,10 @@ require 'resolv'
 describe "Resolv#getnames" do
   platform_is_not :windows do
     it "resolves 127.0.0.1" do
-      res = Resolv.new([Resolv::Hosts.new])
+      hosts = Resolv::Hosts.new(fixture(__FILE__ , "hosts"))
+      res = Resolv.new([hosts])
 
-      names = res.getnames("127.0.0.1")
-      names.should_not == nil
-      names.size.should > 0
+      names = res.getnames("127.0.0.1").should == ["localhost", "localhost4"]
     end
   end
 end