Set VERBOSE to true in test_sni.

The test no longer relies on the value of $VERBOSE being set by outside
environment.

The test test_sni in test_https.rb file checks stderr output for
information about primes being generated.
Since commit 6cb9bf6 the output is only
written if $VERBOSE is true. This happens when `--verbose` is provided
to the Ruby run arguments, or when the tests are ran via `rake`.

test/webrick/test_https.rb

@@ -54,6 +54,9 @@ def test_sni
       # catch stderr in create_self_signed_cert
       stderr_buffer = StringIO.new
       old_stderr, $stderr = $stderr, stderr_buffer
+      # create_self_signed_cert only writes to stderr if $VERBOSE is true.
+      # Set it here to not rely on arguments supplied to the ruby binary
+      old_verbose, $VERBOSE = $VERBOSE, true
 
       begin
         vhost_config1 = {
@@ -85,6 +88,8 @@ def test_sni
       ensure
         # restore stderr
         $stderr = old_stderr
+        # restore $VERBOSE
+        $VERBOSE = old_verbose
       end
 
       assert_match(/\A([.+*]+\n)+\z/, stderr_buffer.string)