test_sni: Remove assert for specific stderr output.

test_sni should be testing SNI use, it doesn't need to
check for specific stderr output which isn't shown by default.

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`.

$stderr is still caught, but discarded to not pollute test run output
in the case $VERBOSE is true.

Author: jackorp

test/webrick/test_https.rb

@@ -87,7 +87,6 @@ def test_sni
         $stderr = old_stderr
       end
 
-      assert_match(/\A([.+*]+\n)+\z/, stderr_buffer.string)
       assert_equal("master", https_get(addr, port, "localhost", "/localhost"))
       assert_equal("master", https_get(addr, port, "unknown", "/unknown", "localhost"))
       assert_equal("vhost1", https_get(addr, port, "vhost1", "/vhost1"))