test/net/http/test_https.rb (test_get_SNI_failure): stop proxy settings

Because the test fails under HTTP proxy settings.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/solaris10-gcc/ruby-master/log/20191210T000004Z.fail.html.gz
```
  1) Failure:
TestNetHTTPS#test_get_SNI_failure [/export/home/users/chkbuild/cb-gcc/tmp/build/20191210T000004Z/ruby/test/net/http/test_https.rb:81]:
[OpenSSL::SSL::SSLError] exception expected, not #<Net::HTTPServerException: 403 "Forbidden">.
```

The new SNI feature introduced at 54072e329c may need to be improved for
HTTP proxy environment.

1 files changed, 19 insertions, 0 deletions

diff --git a/test/net/http/utils.rb b/test/net/http/utils.rb
index dbfd112f31..53f3be0b88 100644
--- a/test/net/http/utils.rb
+++ b/test/net/http/utils.rb
@@ -107,4 +107,23 @@ module TestNetHTTPUtils
     def print(*args) end
     def printf(*args) end
   end
+
+  def self.clean_http_proxy_env
+    orig = {
+      'http_proxy'      => ENV['http_proxy'],
+      'http_proxy_user' => ENV['http_proxy_user'],
+      'http_proxy_pass' => ENV['http_proxy_pass'],
+      'no_proxy'        => ENV['no_proxy'],
+    }
+
+    orig.each_key do |key|
+      ENV.delete key
+    end
+
+    yield
+  ensure
+    orig.each do |key, value|
+      ENV[key] = value
+    end
+  end
 end