test/net/http/test_http.rb: relax open_timeout limit

The test fails randomly on the CI of OpenCSW SPARC Solaris 10:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable10s/ruby-master/log/20200321T091909Z.fail.html.gz
```
  1) Failure:
TestNetHTTP_v1_2_chunked#test_timeout_during_HTTP_session [/export/home/rubyci/unstable10s/tmp/build/20200321T091909Z/ruby/test/net/http/test_http.rb:575]:
[Net::ReadTimeout] exception expected, not #<Net::OpenTimeout: execution expired>.
```

The environment uses RUBY_TEST_SUBPROCESS_TIMEOUT_SCALE=20, so the open
timeout 0.1 sec. means 2.0 sec. for the environment, but it seems too
strict because the environment is painfully slow.

Author: mame

test/net/http/test_http.rb

@@ -569,7 +569,7 @@ def test_timeout_during_HTTP_session
 
       conn = Net::HTTP.new('localhost', port)
       conn.read_timeout = EnvUtil.apply_timeout_scale(0.01)
-      conn.open_timeout = EnvUtil.apply_timeout_scale(0.1)
+      conn.open_timeout = EnvUtil.apply_timeout_scale(1)
 
       th = Thread.new do
         assert_raise(Net::ReadTimeout) {