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.
	---
	 test/drb/test_drb.rb    | 4 ++--
	 test/drb/test_drbssl.rb | 2 +-
	 2 files changed, 3 insertions(+), 3 deletions(-)