Fix stdlib test for Resolv::Hosts by removing /etc/hosts dependency

[pocke]

This PR removes dependency to /etc/hosts content from Resolv::Hosts stdlib test.

Problem

Resolv::Hosts test depended on /etc/hosts content. The test failed on my environment because /etc/hosts is empty.

$ cat /etc/hosts
# Static table lookup for hostnames.
# See hosts(5) for details.

$ bundle exec ruby -Ilib bin/test_runner.rb test/stdlib/resolv/Hosts_test.rb
⚠️⚠️⚠️⚠️ stdlib test assumes Ruby 3.0 but RUBY_VERSION==3.1.0 ⚠️⚠️⚠️⚠️
Loaded suite bin/test_runner
Started
..E
===============================================================================
Error: test_getaddress(ResolvHostsInstanceTest): Resolv::ResolvError: /etc/hosts has no name: localhost
/home/pocke/.rbenv/versions/trunk/lib/ruby/3.1.0/resolv.rb:220:in `getaddress'
/home/pocke/ghq/github.com/ruby/rbs/test/stdlib/test_helper.rb:84:in `block (2 levels) in wrapped_object'
/home/pocke/ghq/github.com/ruby/rbs/test/stdlib/test_helper.rb:247:in `assert_send_type'
test/stdlib/resolv/Hosts_test.rb:24:in `test_getaddress'
     21:   end
     22: 
     23:   def test_getaddress
  => 24:     assert_send_type "(String) -> String",
     25:       hosts, :getaddress, "localhost"
     26:   end
     27: 
===============================================================================
.E
===============================================================================
Error: test_getname(ResolvHostsInstanceTest): Resolv::ResolvError: /etc/hosts has no address: 127.0.0.1
/home/pocke/.rbenv/versions/trunk/lib/ruby/3.1.0/resolv.rb:245:in `getname'
/home/pocke/ghq/github.com/ruby/rbs/test/stdlib/test_helper.rb:84:in `block (2 levels) in wrapped_object'
/home/pocke/ghq/github.com/ruby/rbs/test/stdlib/test_helper.rb:247:in `assert_send_type'
test/stdlib/resolv/Hosts_test.rb:34:in `test_getname'
     31:   end
     32: 
     33:   def test_getname
  => 34:     assert_send_type "(String) -> String",
     35:       hosts, :getname, "127.0.0.1"
     36:   end
     37: 
===============================================================================
.
Finished in 0.003496298 seconds.
-------------------------------------------------------------------------------
6 tests, 12 assertions, 0 failures, 2 errors, 0 pendings, 0 omissions, 0 notifications
66.6667% passed
-------------------------------------------------------------------------------
1716.10 tests/s, 3432.20 assertions/s

Solution

Resolv::Hosts.new receives a path to the hosts file, the default is /etc/hosts. This patch creates a tempfile instead of /etc/hosts.