SSL certificate verify GitHub action (#13697)

* Implement SSL/TLS certificate checking #782

* SSL: Add nimDisableCertificateValidation

Remove NIM_SSL_CERT_VALIDATION env var
tests/untestable/thttpclient_ssl.nim ran successfully on Linux with libssl 1.1.1d

* SSL: update integ test to skip flapping tests

* Revert .travis.yml change

* nimDisableCertificateValidation disable imports

Prevent loading symbols that are not defined on older SSL libs

* SSL: disable verification in net.nim

..when nimDisableCertificateValidation is set

* Update changelog

* Fix peername type

* Add define check for windows

* Disable test on windows

* Add exprimental GitHub action CI for SSL

* Test nimDisableCertificateValidation

tests/untestable/thttpclient_ssl_disabled.nim

@@ -0,0 +1,40 @@
+#
+#            Nim - SSL integration tests
+#        (c) Copyright 2017 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+## Warning: this test performs external networking.
+## Compile and run with:
+## ./bin/nim c -d:nimDisableCertificateValidation -d:ssl -r -p:. tests/untestable/thttpclient_ssl_disabled.nim
+
+import httpclient,
+  net,
+  unittest,
+  ospaths
+
+from strutils import contains
+
+const expired = "https://expired.badssl.com/"
+
+doAssert defined(nimDisableCertificateValidation)
+
+suite "SSL certificate check - disabled":
+
+  test "httpclient in insecure mode":
+    var ctx = newContext(verifyMode = CVerifyPeer)
+    var client = newHttpClient(sslContext = ctx)
+    let a = $client.getContent(expired)
+
+  test "httpclient in insecure mode":
+    var ctx = newContext(verifyMode = CVerifyPeerUseEnvVars)
+    var client = newHttpClient(sslContext = ctx)
+    let a = $client.getContent(expired)
+
+  test "net socket in insecure mode":
+    var sock = newSocket()
+    var ctx = newContext(verifyMode = CVerifyPeerUseEnvVars)
+    ctx.wrapSocket(sock)
+    sock.connect("expired.badssl.com", 443.Port)
+    sock.close