test/drb/test_drb.rb: Specify the host of DRbServer

to try fixing the following error.

http://rubyci.s3.amazonaws.com/opensuseleap/ruby-master/log/20210407T063004Z.log.html.gz
```
[  605/21105] DRbTests::TestDRbSSLAry#test_06_next/home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/lib/drb/drb.rb:1138:in `method_missing': undefined method `regist' for [1, 2, "III", 4, "five", 6]:Array (NoMethodError)
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/lib/drb/extserv.rb:21:in `block in initialize'
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/.ext/common/monitor.rb:202:in `synchronize'
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/.ext/common/monitor.rb:202:in `mon_synchronize'
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/lib/drb/extserv.rb:20:in `initialize'
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/test/drb/ut_array_drbssl.rb:35:in `new'
	from /home/chkbuild/chkbuild/tmp/build/20210407T063004Z/ruby/test/drb/ut_array_drbssl.rb:35:in `<main>'
 = 100.05 s
```

Here is my analysis:
The test of drb used both `druby://:0` and `druby://localhost:0` for
DRbServer. However, the former listens on IPv4, and the latter does on
IPv6, depending on environments. The port 0 is automatically assigned,
but sometimes the same port is used to both because they are different
protocols (IPv4 and IPv6). In this case, their URIs are resolved to the
completely same one (`druby://localhost:port`), which confuses the
method `DRb.here?` which determines the DRbObject is remote or local.

This changeset uses `druby://localhost:0` consistently.

Author: mame

test/drb/test_drb.rb

@@ -323,7 +323,7 @@ class BO < BasicObject
   end
 
   def test_any_to_s
-    server = DRb::DRbServer.new('druby://:0')
+    server = DRb::DRbServer.new('druby://localhost:0')
     server.singleton_class.send(:public, :any_to_s)
     assert_equal("foo:String", server.any_to_s("foo"))
     assert_match(/\A#<DRbTests::TestDRbAnyToS::BO:0x[0-9a-f]+>\z/, server.any_to_s(BO.new))
@@ -335,7 +335,7 @@ def test_any_to_s
 
 class TestDRbTCP < Test::Unit::TestCase
   def test_immediate_close
-    server = DRb::DRbServer.new('druby://:0')
+    server = DRb::DRbServer.new('druby://localhost:0')
     host, port, = DRb::DRbTCPSocket.send(:parse_uri, server.uri)
     socket = TCPSocket.open host, port
     socket.shutdown

test/drb/test_drbssl.rb

@@ -34,7 +34,7 @@ def start
         [ ["C","JP"], ["O","Foo.DRuby.Org"], ["CN", "Sample"] ]
     end
 
-    @server = DRb::DRbServer.new('drbssl://:0', manager, config)
+    @server = DRb::DRbServer.new('drbssl://localhost:0', manager, config)
   end
 end