Use fixture certificates to fix test failure with RHEL9.

http://rubyci.s3.amazonaws.com/rhel9/ruby-master/log/20240711T213004Z.fail.html.gz

```
  1) Error:
HTTPSProxyTest#test_https_proxy_ssl_connection:
OpenSSL::X509::CertificateError: invalid digest
    /home/chkbuild/chkbuild/tmp/build/20240711T213004Z/ruby/test/net/http/test_https_proxy.rb:63:in 'OpenSSL::X509::Certificate#sign'
    /home/chkbuild/chkbuild/tmp/build/20240711T213004Z/ruby/test/net/http/test_https_proxy.rb:63:in 'HTTPSProxyTest#test_https_proxy_ssl_connection'
```

Author: hsbt

test/net/http/test_https_proxy.rb

@@ -44,6 +44,11 @@ def test_https_proxy_authentication
     }
   end
 
+
+  def read_fixture(key)
+    File.read(File.expand_path("../fixtures/#{key}", __dir__))
+  end
+
   def test_https_proxy_ssl_connection
     begin
       OpenSSL
@@ -53,14 +58,8 @@ def test_https_proxy_ssl_connection
 
     tcpserver = TCPServer.new("127.0.0.1", 0)
     ctx = OpenSSL::SSL::SSLContext.new
-    ctx.key = OpenSSL::PKey::RSA.new 2048
-    ctx.cert = OpenSSL::X509::Certificate.new
-    ctx.cert.subject = OpenSSL::X509::Name.new [['CN', 'localhost']]
-    ctx.cert.issuer = ctx.cert.subject
-    ctx.cert.public_key = ctx.key
-    ctx.cert.not_before = Time.now
-    ctx.cert.not_after = Time.now + 60 * 60 * 24
-    ctx.cert.sign ctx.key, OpenSSL::Digest::SHA1.new
+    ctx.key = OpenSSL::PKey.read(read_fixture("server.key"))
+    ctx.cert = OpenSSL::X509::Certificate.new(read_fixture("server.crt"))
     serv = OpenSSL::SSL::SSLServer.new(tcpserver, ctx)
 
     _, port, _, _ = serv.addr
@@ -92,4 +91,3 @@ def test_https_proxy_ssl_connection
     assert_join_threads([client_thread, server_thread])
   end
 end if defined?(OpenSSL)
-